Browse to your steam folder, steamapps/common/construct2
start construct2.exe there. Steam will register it as free version, but when it runs, it will be the registered version. Additionally to that you can run the Construct 2 personal from steam. This way you can at least run two instances.
If that doesn't work, then it has something to do with symbolic links I guess, since I've made one for construct. Use the mklink command or try the steam mover for that task.