The installation of the Cubefree package follows standard GAP rules, see
also 
Chapter Installing a GAP Package in the GAP reference manual.
So the standard method is to unpack the package into the pkg
directory  of your GAP distribution.  This will create an cubefree
subdirectory. 
To use the Cubefree Package you have to request it explicitly. This  is
done by calling LoadPackage like this:
gap> LoadPackage("Cubefree");
Loading Cubefree 1.16 ...
   - Construction Algorithm for Cubefree Groups, 1.16 -
   ---- Heiko Dietrich, heiko.dietrich@monash.edu -----
true
cubefree manual