I think you right-clicked on the wrong thing so I'll try this again with some pictures as I think it'll help me just as much as you.
The first screen shot is from my laptop's "my computer" screen. I've expanded the Music library for this. While the names are mostly obscured, you'll see I have three folders in that Library: My Music (might just be Music for you), Public Music, and another one called Music that's part of a network drive. You will likely only have the first two.
Right click just the Music folder. Minus all the excess crap I have installed you should see a menu that contains the text "Remove location from library". Don't click that, I"m just using it as a reference so if you see it you've right-clicked on the correct folder. You will want to click on properties here.
Then you should get the properties menu that I described in my previous post. It'll looks similar to this: