I am speaking as a layman who knows biology on a general level. The way I understand it, the term microevolution refers to small changes in allele frequency due to mutation and natural selection within a population. Macroevolution refers to a kazillion small changes over long periods of time giving rise to changes significant enough to be noticed.
I guess to me the two terms are analogous to walking. If I walk down the street, when does "micro"walking becomes "macro"walking? Do I have to walk to the store down the street for it to be macro? Do I have to walk to the next town for it to be macro? In other words, I see the two terms, macro and microevolution, as essentially referring to the same process, which is just small changes in the allele frequency.
Another analogy I could think of is the evolution of language. Exactly when and how did old english become modern english? Exactly when did certain aspects of latin break away and gave rise to the romance languages such as french, italian, and spanish?
So, when the creationist demand to see a transition that looks something like a creature with a dog's head, a cat's tail, a frog's legs, and a snake's scale, he's being unfair. It's unfair because that's not how evolution works, much the same way as that's not how evolution of language works. The small very gradual changes over long periods of time accumulated and gave rise to a whole new language.
We could also look at the evolution of technology and see essentially the same thing. Was there an exact year, month, day, hour, or second when the world decided to change from the stone age to the bronze age? Was there an exact year, month, day, hour, or second when the world all in one voice decided to change from the bronze age to the iron age? What sort of tool should be considered a transition between a bronze age tool and an iron age tool? Is a computer a modern or post modern machine?
Now, remember that I am a layman in terms of evolution. Corrections are welcome