I use the "Merge to current" button of the tkcvs tree view to merge
changes from  one branch to another. After clicking on the button, the
"tag and commit" dialog appears. I click "I'm ready" and edit the
"merge from" tag to "mergefrom_tkcvs-bug_18Jan10". After clicking on "Ok", I
get the cvs output and see that my changes are gone and have been
replaced by tkcvs's default:

cvs commit -m "Merge from 1.67.4.2" "make_release"
cvs tag -F -r1.67.4.2 mergeto_trunk_18Jan10 "make_release"
cvs tag -F mergefrom_branch-0-2-20-Y_18Jan10 "make_release"
T make_release

I'll try to attache some pictures later.