You should have a program under Applications called CtlgHelp. This stands for Catalog Help. Go to it, press enter, then press enter once again. Then when you go to 2nd Distributions and highlight "tcdf", press the "+" sign. It will then give you the parameters required. (If you go to tcdf and press enter, it's too late to use this feature.)
If I were you, I would make sure that every student has this function turned on. A TI-83 cannot take this program -- insufficient memory. The TI-83 Plus can take this program, but you might have to delete a couple of other APPS to make enough memory. Even if yours does not have this program (I just noticed by TI Nspire does not), you can link to any calculator that has it and copy.
Stacy Elliott wrote: When using the "tcdf" on the TI-84, what are the values you put in? Is it "tcdf(lower bound, upper bound, df)"? or "tcdf(lower bound, upper bound, sample size)"? Or, something different altogether???