I prefer a slight modification to Rob's script, which prints the name of the project as well. This allows one to run it with just an action selected and get the full name, useful in cases where you don't have the project name displayed.
Code:
property pTitle : "Show folder path"
property pVer : "0.03"
tell application id "OFOC"
tell front document window of front document
repeat with oPanel in {content, sidebar}
set lstValue to value of (selected trees of oPanel where class of its value is project or class of its value is task)
if length of lstValue > 0 then exit repeat
end repeat
if length of lstValue < 1 then return
display dialog my FolderPath(first item of lstValue) buttons {"OK"} default button "OK" with title pTitle & " ver. " & pVer
end tell
end tell
on FolderPath(oNode)
set str to ""
tell application id "OFOC"
set oParent to container of (containing project of oNode)
repeat while class of oParent = folder
set str to name of oParent & ":" & str
set oParent to container of oParent
end repeat
set str to str & name of (containing project of oNode)
end tell
return str
end FolderPath