The most generic way to load the library is:
:- lib(eplex).This will try to load an appropriate external solver available on the computer.
It is also possible to request a specific solver explicitly, see section 11.11 for details.
Note that the ECLiPSe library described here is just an interface to an external solver. In order to be able to use it, you need to have access to a solver supported by the library. For commercial solvers, this may require a licence for the solver on your machine.