trigger to compare files

[yoose]

Hi, I would like to select two files and use a trigger to open and
compare them either in FileMerge or DiffMerge. I brought them into
quicksilver and did an open with, but it did not work. I also tried
the FileMerge services from finder with two files selected, but all it
does is open the FileMerge dialog without the files selected. Any one
have any ideas? Thanks.