Once you create the tangent, you can hit the Flip button to flip it to the other side if it is not on the side of the plane that you want.
Once you create the tangent relationship, you will be put into the process of creating the next relationship. At that point, go to the dropdown list of relationships on the command bar (the list next to the relationship type button where you selected the tangent relationship). Select the tangent relationship you just created and a ribbon will appear with the 'Flip' button on it. Starting with ST7, that ribbon will appear automatically. The other way you could do it is exit the command to get back to Select Tool once you have created the tangent. Then, select either of the parts and select the tangent relationship in the lower portion of the Pathfinder once it appears. At that point, a ribbon will appear in the graphic view that has a few things you can edit for the relationship - there is a Flip button on there as well. You can also RMB on the relationship and there is a Flip option on the context menu.
Hope this helps,