Problem with diff tool

Hello,
I have installed DiffMerge (64 bit) on my Win7(64 bit) machine. But, Unity does not detect it. I tried re-installing several times. Why is this happening?

I had both P4Merge and TortoiseMerge installed on my machine prior to installing Unity on a Windows 7 x64 machine yet Unity cannot find them. From what I gather, this is a problem with 64 bit Windows but I have yet to figure out exactly how to fix it.