When I try to run Idplot it gives me a "command not found" error in the terminal. How can I solve this?
Are sure you are typing
1dplot, that is, which starts with the number "one"? That is the actual name of the AFNI program.
From copying and pasting what you wrote here, it looks
Idplot, starting with a "capital i".
Online page of the 1dplot program help, for clarity.