Auto-complete is temporarily absent from Numbers 3, due back within a few months according to Apple.
If you frequently enter repeating values in a column and don't want to type the same text over and over again, note that Numbers 3 has an enhanced Pop-Up Menu functionality that in many sitiuations is a better choice than "auto-complete" since it helps prevent typos or capitalization inconsistencies from creeping in and messing up your data. It is also much more efficient than auto-complete in Numbers for iOS, should you ever want to sync to an iPad or iPhone and do data entry there.
Pop-Up Menu is much enhanced over what it was in Numbers 2.3, and is easy to set up:
Select the whole column or range before formatting as Pop-Up Menu and your existing values in that column or range will pre-populate a menu automatically:
Then you can remove the values you don't want in the list, such as the column header (and possibly some previous spelling errors too!):
And after this easy one-time setup all you have to do thereafter is to choose from the pop-up list when you add rows:
See this post for more details on how this handy feature has been enhanced in Numbers 3.0.
Should you ever want to take your Numbers with you and do data entry on an iPad or iPhone, in a touch interface, pop-ups are much more efficient than typing the first few letters of items. You can scroll with ease through a list of dozens of items to make a pick. Anybody using Numbers cross-platform should favor Pop-Up so they can use their documents efficiently for data input on the go, which can be a productivity booster.
See this thread for this and other workarounds for Numbers 3.0.