In Simple Entry, enter the first note. Add the notes above (or below) by tapping the number key for the interval. For example, for C-E-G-C, you would enter C 3 3 4. Use Shift-interval to go down.
I use the numberpad. You will have to check the appropriate keys if you don’t have one, like on a laptop.
As Peter says, we need more info from you. Also, many “it used to work but now it doesn’t” can be fixed with a computer restart. If that doesn’t do it, recreate the Finale preference file. See your manual for instructions.