Thanks for the report. I verified with a couple of InterMapper releases that this entry field still exists.
Please be sure you're opening the Info window for the link (at the circle at the end of the link line) rather than the Info window for the device itself.
Adding networks to an interface is allowed, but not to the device itself.
If I'm misunderstanding what the issue is, please use the Support > Submit new case link above to send further information, including InterMapper release, and if possible a screenshot of what you're seeing and we'll open a support case. Thanks,