If you've already entered players in teams but you still can't enter stats when you enter results, then make sure that your stat types haven't been hidden or archived.
For individual stat types go to Setup > Statistic Setup > Individual Stat Types.
Edit the stat type and uncheck the 'Hidden' and / or 'Archived' boxes.
If your leagues uses Game Formats go to Setup > Statistic Setup > Game Formats.
Edit the game format, then edit the game group and uncheck the 'Archive' box.