(In response to a question from Tom Phillips....)
You are expected to add the descriptive monster text to your Round 3 submission--just like in a Bestiary entry.
(Ross just added a placeholder line on the sample stat block for that purpose.)
The descriptive text counts toward your R3 submission's word count limit! However, you can revise your monster descriptions (as the R3 rules say), and in many cases you'll be able to strip out rulesy stuff from the description by using the appropriate game text shorthand. For example, you don't need to say your creature is "about the size of a tiger, very strong, and as fast as a horse," you just put "Large" in the size section of the stat block, "Str 20," and "Speed 50 ft.," which should save you some words.
Don't worry, next round you'll have a MUCH larger word count to play with. :)