If you havent allready, get authoring tools Here!
Also, I know you could just acsess the authoring tools through the Tools section in steam, but whatever. Its cool to have!

In the bottom left corner of the screen in the library, It should say Add A Game. Now hold on, I see you hovering over Add a Steam Game. Press add a Non-Steam game.

You: This makes no sense!!!

Explanation for skeptic people: Trust me.

Press browse, and locate your Portal 2 folder.
On Windows 64 bit, it should be at C:/Program Files(x86)/Steam/SteamApps/common/Portal 2
On Windows 32 bit, it should be at C:/Program Files/Steam/SteamApps/common/Portal 2
On anything else, well thats a Google question. Cant help you there.

Usually in there, should be something peculiar with a name like Authoring Tools Beta.exe or something. I can't be sure, litterally. But be honest, you have the mental capacity to do realise wich one it is. In this image, there is none. Thats what happens when you don't follow my note. When you select it, Press Open, and hallalalalyah! Done! There is still some polishing to be done though...

FAQ:
Q. Why is it crashing?
A. This is most likely not prior to theese steps. I cannot
help you with a problem not accociated with this, unless you really need an explanation.

Q. Can I do this with other Non-Steam Games too?
A. Of course! You also will not have to right click the file in the library and tell steam what program to use to open it. This is a fully automatic process.

Q. What source did you get this from?
A. My common sense and trial and error.

(Just for humor, not a question)
Q. Why that picture?
A. Well... I don't know. Maybe I am a sentient race of non-brainy statues. I probabaly come from MT. Rushmore.

(To reassure you, theese are humane questions)
Q. Can I use theese exact words to explain to my friends?
A. No copyright. Im just a gamer. Im okay. Geez, so many online concernios.

Q. Failed to run game (Missing executable) error?
A. Go to the Portal 2 folder, and delete it. Go to the library, and right click it, then press delete local content. Then follow my little note I wrote in the beggining.