Your Borderlands2 Application file version (Borderlands2.exe) needs to be the same as the Trainer version.
For me, that file is located at:
C:\Program Files (x86)\Steam\steamapps\common\Borderlands 2\Binaries\Win32\Borderlands2
That last Borderlands2 is the .exe file. You can hover your mouse over it, or right click it and then click properties, in order to see the file version (which is different from the product version, by the way).