I created an add-on with a Dexterity content type Task using mr.bob (with Plone 4.3.6 as option). I created several instances of Task inside a Plone site. In a page I tried to create a link to one of the tasks inside a RichText field with the Insert/Edit tool of TinyMCE. However, content items of type Task does not appear in the UI that navigates the directory tree. I think that I have to add the behavior plone.app.referenceable to the Task content type. However, adding such behavior does not help. I guess that it is an issue.
TinyMCE needs to be configured for the new type to be linkable. Check out the tinymce configuration settings.