Go into your first level model DSGN file and add this after your DSGN.conditions section. This references your conditions. You can indicate (with a 1) if you want that condition to be modeled as single trials or not. Easy!
You think your edits are golden and should be shared with everyone? Use 'commit' and give a little message about what you have changed. Important: make sure you check your status. You will commit everything you have changed, so revert things you do not want to commit.
svn commit -m "Added info on single trial models to the README -marianne"