食べます or 食べています
Hi ! I'm also learning Japanese with Rosetta Stone, and when they had "to eat" or a sentence in which someone was eating, it was always spoken and written as 食べています. Duolingo has it without the てい. Do you need the てい ? Does it change the sentence in any way ?
They are two different forms of a verb:
食べます (tabemasu) - plain polite form, meaning "eat", "will eat"
食べています (tabete imasu) - -て form, meaning "is eating" right now, "is in state of eating"
Don't restrict yourself to learning just through Rosetta, it doesn't really explain grammar. If you want to understand Japanese, you should use other resources for that.
Hope that will help you! :)
Edit: that form appears in Duo course too, just a bit later ;)