You should also note that the unionInclude.pg macro is deprecated, and will cease to function with the next version of webwork2 and pg. Instead use includePGproblem. The macro to load to use that method is PGstandard.pl. To use that to obtain a random problem use
@choices = ('DerivateEasy/enkelt1.pg', 'DerivateEasy/enkelt3.pg', 'DerivateEasy/enkelt4.pg', 'DerivateEasy/enkelt5.pg');
You should check the paths of those files though. Are those relative to the courses templates directory? If they are not that won't work for either the unionInclude.pg macro or with the includePGproblem method.