Are you trying to do a join in Sql, or trying to use the data blending feature in Tableau.
If the former, could you post some sample data for the 2 tables.
If the latter, could you post a sample workbook in .tbwx (packaged workbook format) Do a save as packaged workbook.
And btw, very little of this stuff is hilariously obvious. The first time I used Tableau I spent 6 hours trying to get a distinct count out of a .xls data connection. Drove a buddy of mine who's a dbase programmer crazy asking questions. Then I discovered that you can't do a CountD in .xls unless you first create a data extract. Oh yea, that opens up some additional aggregations for measures not otherwise available.
Stick w/ these frustrations (i wish we didn't have to deal with them) and it becomes worth it. Fellow users are very helpful. It's a great community.