There are two possible reasons for not being able to add a MIB to Modules:
MIB is dependent on other MIBs
MIB has a syntax error
MIB can only be added to Modules if all MIBs that it is dependent on are already added in the Modules. Application will inform you of the list of missing MIBs. You need to download all the missing MIBs from the list and add them before trying to add the desired MIB again.
In some cases the MIB file can contain syntax error(s) that does not allow the application to pars it. You can try to fix the file your self, or rise a support case by sending an email to firstname.lastname@example.org.