Then I will play devil's advocate, any evolutionist can jump in at any time and correct me if they have alternate answers to this question:
Evolutionists believe that lots and lots of micro-evolution over time causes macro-evolution. The reason we cannot macro-evolution happening it is because it takes many micro-evolutionary steps in order to form a completely new organism that is distinct from it's ancestor, and other organism from the same ancestor whose evolutionary path diverged. Essentially, they would say that the long time periods needed for this process to occur would make it impossible to observe examples of an organism tranforming into another classifiably different organism.
This is basically correct, but I'll still put it in my own words for whatever that might be worth to you.
First, I want to say that I reject the differentiation between micro and macro evolution, but I will roll with it because I fully understand what you mean by it. I am going to start with a very basic description of biological evolution, so bear with me:
Biological evolution consists of three separate but related processes: Inheritance, Variation, and Selection.
Inheritance is obvious, and I don't think any of you guys would object to it. It is the reason that children look a lot like their parents, it is the reason Asian parents have Asian looking babies, European parents have European looking babies, etc.
Variation occurs in a few different forms. The most common is genetic recombination in organisms that reproduce s*xually. Both the father and the mother contribute their genetic material to the child, "mixing it up" if you will (I could go into the details but it isn't necessary here). Variation also occurs via mutation, which is basically a transcription error made while copying the genetic material. Most mutations are benign or detrimental, as would be expected by a random error. However, occasionally some prove to be beneficial to the organism, which you would also expect.
Selection is the simple fact that, on average, individual organisms better suited to their environment will reproduce more frequently than those less suited to their environment. This of course makes perfect sense, and I don't think anyone has a problem with it. When you combine this fact with inheritance and variation you get a gradual trend toward genetic variations (alleles) that are better for the organisms survival and reproductive fitness. I consider this an obvious conclusion but if you have any questions about why this is true please ask
One more thing you need to know is what a population is. A population is a group of organisms that interbreed, that's all. If you have two organisms and there is a chance that they could breed with each other and produce viable offspring then those two organisms are in the same population... simple right?
Now, to answer your question, biological evolution is about the change in the genetic material of a population over time. This can and usually does occur slowly, across many generations. It is also possible (though less likely) that a random mutation could be very beneficial, in which case this mutation would spread through the population relatively quickly (but still on the order of tens to hundreds of generations, potentially thousand of years depending on the rate of reproduction, the size of the population, the degree of the advantage provided by the mutation, and the selection pressure, or how difficult it is to survive).
What you call micro evolution I presume to be changes that are not obviously apparent to an observer, while macro evolution would be very obvious changes to the organism, or even speciation (correct me if I am wrong). If that is the case then micro evolution occurs constantly, and macro evolution would be the net sum of a relatively long period of micro evolution OR the result of a beneficial mutation. Speciation, which you would also call macro evolution, occurs when a population splits into two or more populations. When this occurs the new populations that used to be one no longer share the same gene pool, so beneficial variations in one are not shared with the others. Over time, these changes (due to mutation or simply recombination) accumulate and the populations begin to diverge physiologically. If enough change occurs from one population to another it may become physically impossible for them to ever interbred again. We see the beginning of this process today in Mules, which are the offspring of Donkeys and Horses. Donkeys and Horses are defined as different species, even though they can reproduce and make Mules, but the Mules are almost always infertile, meaning they cannot continue their lineage. SOMETIMES a mule is able to mate with another mule, but due to genetic incompatibility this is rare. Horses and Donkeys are on the verge of becoming completely incapable of breeding and creating fertile offspring, but they aren't quite there yet. Because of this they are a remarkable modern example of speciation in progress.