First of all, you don't point to any DLL. ForteDXi is a DXi synth, not a VST synth. As such it is registered in the Windows Registry, which knows about it. When you launch BIAB (32-bit only, it will not work in 64-bit BIAB), BIAB scans the registry for available DX/DXi plugins/synths and builds the list of available plugins/synths.
Make sure you launch BIAB "as administrator", so that the activation feature gains that right to write your license information to the registry.
If you don't see ForteDXi as a selection, you might need to rescan for new DXi's (especially if you just re-installed it).
See
this post from Kent at PG Music. Maybe it will help.
If you need to re-enter your serial number, once you see Forte DXi in the list of avaliable synths, select it, then right click on the synth interface and select register. Enter you serial number and hit OK (you'll have to click on OK twice). Close BIAB, relaunch, and it should be available and authorized/activated.