To launch the terminal version of the program type math. There is a hypertext help system, which includes an online version of the Mathematica manual, available from within the program. To setup your account to use IMSL 2.1, use the CAEN swselect utility and select version 2.1 as yor default. This will make all your UNIX shells ready to compile and run IMSL 2.1 programs. Alternatively, to setup your current shell to run IMSL, type imsl at the UNIX prompt.
All documentation is available online.