If your rail is off to the right (looking through the scope) it can be corrected.
I had the same problem when I first got my TAC 15
. The bow was assembled and had dust on it from sitting a log time. I think someone had dropped it and moved the rail. I used three pieces of .005' thick brass shim stock between the Optimizer base and the rail on the left side of the rail. You can see the bright shinny brass folded over the rail in the center of the photo below. If you need shim stock, PM me your mailing address.
The second photo is looking down the barrel (through the drop-away rest) at the end of the rail and the end of the Optimizer base just under the scope to give you an idea of how far mine was off. You can see the Optimizer base and the rail aren't close. Like you, I was really frustrated; but don't give up, it can be made to work!
I'm sure you know to count the total clicks on your scope both vertical and horizontal, divide that total in half to center your cross hair.
I alos had to use Burris Signature ZEE rings and a POS-ALIGN insert kit to get mine zeroed.
Let us know how you do.
Dorge at Firenock makes the best arrow for the TAC bows, not cheap, but the best!