My environment:
VS 2010 Prof., with no DNN Starter Kit.
DNN site on my local -site working.
Then I install Forum module (source version) from codeplex - module working.

Now I want change something in source of Forum module.
I open VS2010 -> File -> Open WebSite -> File system -> Forum module folder (in DesktopModules folder) - is this correct? What I must change in Property Pages

or

I open VS2010 -File->Open Solution -> Navigate to Forum module folder, and open .sln file
In this solution I lose all my code behind files

How correctly set up my environment to develop this module. (I know how to create new WAP module and I know tutorials from Michael Washington site)

Ok
I have fresh dnn site on my local, then I install Forum module (source version).
After that in VisualStudio File->Open->WebSite->FileSystem->Forum folder (in DesktopMdules)
Add reference to DotNetNuke.dll in bin folder in DNN site

Then after 'Build Website' first error "Could not load type 'DotNetNuke.Modules.Forum.AvatarAdminControl'"
In *.ascx files first line (where is <% Register.... ) second error ForumConfig is not declared

OK. I found solution
1. In VisualStudio File->Open->Project/Solution - *.sln file in DesktopModules/Forum
2. Convert to VS2010 project.
3. Set target .NET Framework to v3.5
4. In Project Properties in Web tab set values according to this http://www.mitchelsellers.com/Portals/1/Articles/Images/WAP_Project2.jpg