Hello Ruben,
it appears you are trying to read a SPE file, which unfortunately is not a supported file type.
So I think, the problem is not an incorrect configuration of IDL_PATH but rather SPE being an unsupported file format.
To circumvent this issue, you could make use of IDL's Python Bridge (
http://www.exelisvis.com/docs/Python.html)
and the SciPy Cookbook (
http://scipy-cookbook.readthedocs.org/) which is able to read SPE files.
Regarding your original question:
Instructions on how to edit IDL's search path can be found here:
http://www.exelisvis.com/...prefs_directory.html
Best regards,
Moritz