I have found that when open any module source into vs2013 all dnn controls report warnings in the ascx files.
Namespace or type specified in the project-level Imports 'DotNetNuke.Common' doesn't contain any public member or cannot be found. Make sure the namespace or the type is defined and contains at least one public member. Make sure the imported element name doesn't use any aliases and so on for all the references. also all elements that are dnn states they are not a known element. Is there a change required in the web.config? Is DNN not supported in vs2013?
The compile fine with no errors.
It appears ok in vs2010. I skipped vs2012