I'm having the same problem. I've spent 3 or 4 hours trying to figure this out, searching everywhere and experimenting. I'm still stuck. I finally sent an email to PG Music Support but haven't heard back yet.

When I installed BIAB 2018 Pro it installed Amplitube 3.

I created an IK Multimedia account and somehow already had software products there (although I've never purchased any) but none of them Ampltube 3, just Amplitube 4 custom shop. When trying to use Amplitube 3 in BIAB I get a prompt to authorize it and the IK Authorization manager pops up. I tried the Amplitube 4 serial number but it didn't work (I didn't think it would).

The only serial number I could find was in C:\bb\Data\TicketsActivated. But it's not an IK Multimedia serial number (it's not in the correct format), but the BIAB 2018 serial for my install. I've scoured my PC looking for another but no luck.

I purchased BIAB 2018 Pro as a digital download, so I guess the Amplitube 3 s/n isn't included in that format.

And I can't use Amplitube 4 without purchasing the JBridge (which I did), because Amp 4 is 64-bit only.

With the amazingly huge number of PG Music FAQs, tips and tricks, and amount of info in the PG Forums, I'm surprised there isn't anything to help with this. I would think others have asked about this. Maybe AMP 3 is old school and nobody uses anymore.

I've given up for now, I just want to get using this software. Although it would really be cool to check out some of the Amplitube 3 demos... ah well, maybe try and convert the few that work in BIAB Pro to Amp 4.

BIAB 2018 / Studio One 3 / Guitar Pro
Intel i5 (7th Gen) 3.00GHz / Win 10 Home 64bit
MBox (3rd Gen) USB / Macki MR5 Monitors