There are four fields for every entry. If the example sentences are like a comment, you can add them to the fourth field.
If you want to test yourself if you can translate the sample sentence correctly however, you can add them to the entry / translation fields, along with the vocabulary, in a second line. Or you can also create an additional entry for the example sentence alone because there is no need to link the vocabulary test with the example sentence translation test.
But in general, adding a sample sentence just helps you to "understand" new entries when learning with the learn dialog (not testing), so I guess they act more like a comment, and the fourth field is ideal for that.