I have just been through this same problem (I think). I'm assuming that you have a blank form and have inserted a Tab Control onto the blank form (if not start that way).
To add feilds to the blank Tab, select the Tab you want to add fields to (by clicking on the Tab Tag (the tag at the top of the Tab control which has the name in it), once selected you can drag fields from the field list across into the tab control (which will turn black).
If you don't do this the fields are added to the form (which look like the tab is transparent).
Hope this helps.