Let is just the base Let language extended.
Proc is the extended let language extended to include procedures, and has some of the procedure related exercises implemented.
LetRec is the extended proc extended to include recursion, and has some of the letrec related exercises implemented.
Open any of the languages inside of DrRacket.
You can either use: (run "YourProgramHere")
or you can use the repl loop to continuously run programs, assuming they have no errors: (repl)