If something has changed in your database (e.g. you've added a table or changed the name of a column), then:
Click the settings icon in the top right of Trevor.
From the dropdown, click Refresh tables metadata.
Trevor will fetch the changes and reflect them in the interface.
If your table or column still doesn't appear, then it's possible that you don't, or Trevor doesn't, have permission to access that table / column. If this is the case, it's best to take this up with your Trevor admin.