Triplet problem

Hi everybody,
I am having an issue with triplets. I am trying to do a quarter note value triplet (3 eighth notes) but it won't let me enter the notes. I select the quarter note rest, press "n", press command 3, a blank triplet pops up in the place that I want, the first note of the triplet is selected but I can't enter any notes. when I try to, it enters a note before the triplet giving me an extra eight note in my measure. when I select the second or third note in the triplet and try to plug in a note, it gives me a pop up saying, "you cannot enter a chord in the tuplet" or something like that (i selected the box for it to stop notifying me in hopes that it might fix the problem). does anybody know how to help? Thanks!