When you install winmerge after TortoiseSVN it gives you the option of associating winmerge with Tortoise. But if we install TortoiseSVN after winmerge how can we associate Winmerge to be used instead of Tortoise default diff viewer.
You can open the TortoiseSVN settings and manually enter it:
My command line from the screenshot above is:
C:\Program Files (x86)\WinMerge\WinMergeU.exe -e -x -ub -dl %bname -dr %yname %base %mine
If you install TortoiseSVN after WinMerge, probably the easiest way would be to uninstall and re-install WinMerge again.