"My salad has tomatoes."
Translation:Η σαλάτα μου έχει τομάτα.
Well as I said above "τομάτες" is one of the accepted words for "tomatoes". However, a sentence is made up of more than one word. So, if yours was not accepted there must have been a mistake somewhere else.
Check out these instructions:
TIPS TO HELP YOU LEARN + HOW TO REPORT A PROBLEM
And check out the Greek Forum here with more links.