According to this: http://msdn.microsoft.com/en-us/library/605a12zt.aspx
"You cannot specify an external calling application in the Express versions. To debug a DLL in an Express version, add an executable project to the solution, set it as the start point for the solution, and call methods in your DLL from the executable project."
I've noticed many people are using the express editions though, how are they coping? There must be a trick to it.
I'm sure if you Googled around a bit you'd probably find a work-around. It is probably the case that the buttons are disabled but the functionality remains in-place. My guess would be that it can be enabled manually by text editing one of the project config files.
5min later...
Oh, I just checked and the setting appears in the user options config file for the project (full version of VS2013). Look for a file in your project folder called something like MyProjectName.csproj.user
If it's there, open it in a text editor see if it looks anything like below.
<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="12.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
<PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Debug|AnyCPU'">
<StartAction>Program</StartAction>
<StartProgram>C:\Program Files\Autodesk\Revit 2015\Revit.exe</StartProgram>
<StartArguments>""</StartArguments>
<EnableUnmanagedDebugging>false</EnableUnmanagedDebugging>
</PropertyGroup>
<PropertyGroup Condition="'$(Configuration)|$(Platform)' == 'Release|AnyCPU'">
<StartAction>Program</StartAction>
<StartProgram>C:\Program Files\Autodesk\Revit 2015\Revit.exe</StartProgram>
<StartArguments>""</StartArguments>
</PropertyGroup>
</Project>
if it's not there, paste the above into a new text file, name it as I described earlier and save it to your project folder. See if that does anything?
You may need to tweak the values in the PropertyGroup tags to match the names of the build configurations in your project if they are different. Also if you want Revit to open with a specific document file, add it's path into the <StartArguments> tags
Now I haven't tested any of this and it may not even work, but its worth a shot!
