Down with FizzBuzz... LOL
I like how you have declared the Fizz and Buzz 'constants'. It makes the code more readable.

The comments are useful too, though, for consistency, the second comment should be changed from:


BTW Froot will run 1 through 100



to

BTW Froot runs 1 through 100


The idea of generating Froot from Frootz is also good, because it reduces the computations later in the code.

I only wish you used a better name than Frootz because it conflicts with Froot. The similarity is.... uncanny.

I would suggest the name Loopz:

I HAS A Loopz ITZ SUM OF Frootz AN 1


This will reduce the ambiguity, and improve the readability and maintainability.

Finally, it is good that you put the Fizz modulo check before the Buzz check because that improves performance. Many people put buzz first, but that means the modulo-5 check happens, and that fails more often than the modulo-3 check, so people who do the Buzz-loop first tend to do about 20% more conditional checks.

All in all, for a first go, I am very impressed with your result. I can't wait until you implement a fim++ interpreter in lolcode. Till then!