A library X can be referenced in a Signal program by the declaration
use X;

The compiler searches the file X.SIG (or X.LIB) using the shell variable SIGNAL_LIBRARY_PATH
You can add your personnal rools using the setenv command.

Example: to add /udd/User/MyLibDirectory, execute

setenv SIGNAL_LIBRARY_PATH "/udd/User/MyLibDirectory:${SIGNAL_LIBRARY_PATH}"