So, the good news is that we talked many of our users into abandoning Microsoft Access in favor of using APEX. Part of this sell was the APEX Query Builder feature. However, in 4.2.1.00.08, the Schema selector has, apparently, disappeared from the Query Builder. The only tables they can see in the QB are from the Parsing Schema.
1. Proper access has been grated to the parsing schema to the other schema objects at the DB level.
2. The other schemas have been added to the Workspace.
3. The other schemas are available in the object browser.
The schema selector was certainly available in previous versions of APEX. See http://docs.oracle.com/cd/E10513_01/doc/appdev.310/e12857/qry_bldr.htm. However, it is gone now.
Good news: The select list for schema selection is actually there on the page.
Bad news: You can't see it.
I've filed Bug 16292676 for this specific issue. We'll take a look at it and get back to you on this thread. However, give us a couple days at least, as we're a bit encumbered on a few other things at the moment.