help with grace notes

Help. I am a newbie and am trying to add grace notes. I see the grace notes on the pallette but I can't seem to place them into the music. Sometimes they just turn into regular notes and create a chord. Other times I can't seem to add them at all. I'll bet that I'm doing something wrong but I just can't figure it out. Any help would be appreciated. I don't know if it matters, but I am using Windows Vista. Thanks.