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.