In Visual Studio, go to the Tools menu, select Options, expand Source Control, (In a TFS environment, click Visual Studio Team Foundation Server), and click on the Configure User Tools button.

Click the Add button.

Enter/select the following options for Compare:

• Extension: .*
• Operation: Compare
• Command: C:\Program Files\Beyond Compare 3\BComp.exe (replace with the proper path for your machine)
• Arguments: %1 %2 /title1=%6 /title2=%7

If using Beyond Compare v3 Professional (3-way Merge):

• Extension: .*
• Operation: Merge
• Command: C:\Program Files\Beyond Compare 3\BComp.exe (replace with the proper path for your machine)
• Arguments: %1 %2 %3 %4 /title1=%6 /title2=%7 /title3=%8 /title4=%9

If using Beyond Compare v3 Standard or Beyond Compare v2 (2-way Merge):

• Extension: .*
• Operation: Merge
• Command: C:\Program Files\Beyond Compare 3\BComp.exe (replace with the proper path for your machine)
• Arguments: %1 %2 /savetarget=%4 /title1=%6 /title2=%7

from : http://stackoverflow.com/questions/4466238/how-to-configure-visual-studio-to-use-beyond-compare