I am currently having issues adding a relatedmedia field type to a content type inside a module. The issue isn't when I add the field, but when I try and edit/create the content and where it says:
Content from this type cannot be selected because the module or the source providing its content is deleted or deactivated.
There is already a field called profileimage of type Media which seems to be working just fine. From what I've read Media is pre sitefinity 7.0 and RelatedMedia is for 7.0 and above.
I looked through some of the other sites that are on this setup, and some of the other content types have RelatedMedia field types set up and working. I have also looked at this G+ page but not sure if this is the culprit.
We are running sitefinity 7.2.5310.0.
When you set up the related media you had a choice for the provider if you didn't select "default site source" I could see why it wouldn't be able to find media items.
Make sure to set "default site source" as your provider if you only want to access that sites media, or select the proper provider for the site you are if you are only using it on one multisite, make sure to only enable the module on that site as well