As far as adding tables, I just right click on the database drilldown on the left side, and select “Create Table…”
To edit a table, left click it in the drilldown on left side, then press F6.
As far as why you can't add data, I see no reason you can't add data simply by entering it into each field, then hitting the save changes button. Can you give more information about the database and table type? Perhaps someone else knows more about what might cause this issue than I do, though.