OnMerge doesn't support 64-bit Word, though 64-bit Windows is fine. Unfortunately, adding 64-bit support is no small matter for our company. Microsoft has decided to not produce a 64-bit version of the development tools we use to create OnMerge, so eventual 64-bit support will require us (and most other addin vendors) to completely rewrite our software from the ground up. We have no plans to do that at this time.
What can you do?
We know of no products similar to OnMerge on the market, can only suggest that you install the 32-bit version of Office instead. This should be a free "side-grade."
32-bit Office works great on 64-bit Windows and, to be perfectly honest, there is currently no benefit of 64-bit Office over the 32-bit one unless you're playing with COLOSSAL (i.e. 2 gigabytes+!) spreadsheets. In our own testing, the 64-bit version loads even slower than the 32-bit one. Microsoft is looking to the future with the 64-bit version, but the actual need/benefit is still many years away for 99.9% of users.