Let me first say, ehab, you have made my day by posting this. I had no idea that you could do this with SlickEdit project files. Very convenient!
Anyway, I didn't have much luck with using environment variables either (also tried %rp for referring to the project path, same thing as ProjectPath in that it wasn't expanded either).
However, I thought of something else that seems to work well for me. My project directory is structured like this:
project_root/
|
|--- project.vpw
|--- project.vpj
|--- src/ (contains *.cpp, *.c, *.h organized in sub dirs)
|--- lib/ (contains sub dirs for other open source *.c/*.cpp/*.h libraries I'm using)
So, I put the following in my .vpj:
<Files AutoFolders="DirectoryView">
<F
N="./src/*.*"
Recurse="1"
Refilter="0"/>
<F
N="./lib/*.*"
Recurse="1"
Refilter="0"/>
</Files>
Which appears to work fine as the current directory "./" appears to be based on where the vpj (or maybe the vpw?) is.
Maybe something like that might work for you? Would be nice either way for environment variables to work here though eventually.