The only contents of the User package are files that you put there or files put there by packages that you’ve installed (for example settings). There’s not a direct interface to manipulate a sublime-mousemap file like there is for sublime-keymap because unlike key bindings it’s not possible to overload mouse bindings with context, and as such you need to create such a file yourself.
You would create that manually by editing the content and then saving the file in your User package. You want the file to be named Default (PLATFORM).sublime-mousemap, where PLATFORM is either Linux, Windows or OSX (and the case of the filename matters depending on the platform you’re on, so it’s best to always use the correct case to forestall potential problems.
It’s stored in the Default package (that’s what the Default/ part of the resource path is specifying). You can view it by using View Package File from the command palette and entering a filter like sublime-mousemap or similar; the file that you open will be read only, but you can use it’s contents for reference purposes.