Where did you buy the CS5 version? Like I said, there is no such thing as Premiere Pro CS6.5. The only "6.5" that ever existed was the old Premiere version (before there was "Premiere Pro"). If your purchased product contains a Premiere Pro version 6.5, then you have been sold a counterfeit copy of Premiere Pro and you should get your money back ASAP.
If you have Premiere (not Pro) version 6.5 (not CS6.5), then you can try to activate CS5. However, I don't hold out much hope because even if Adobe allowed an old Premiere-6.5-to-Premiere-Pro-CS5 upgrade at one point, it was likely a limited-time offer and the current activation servers may not allow it. If you were sold your copy of CS5 with only Premiere 6.5 as the source of validation, you should still try to get your money back, because at this point in time it's likely not a legitimate source of CS5 activation.
Jeff