This was supposed to be a comment, but it turned out too long.
It might be that the trackpad is slightly damaged so that the button is somehow pressed down constantly.
I have a black macbook that clicks every time I rest my hands anywhere near the area below the keyboard or around the trackpad. The plastic around that part of the case has kind of caved in and it affects the trackpad. If something similar has happened to your macbook, only that it is constantly pressing the button, you wouldn't be able to press any buttons with any mouse.
On my own macbook, all of this (the case caving in) started after almost the entire bottom edge of the top case was breaking off, like in the picture below, which is a common problem on many of the macbooks (picture through google).
Disabling the internal trackpad while only using the keyboard
- Open Spotlight. The keyboard shortcut for this is usually
- Once in System Preferences you should be able to select "Accessibility" simply by starting to type it (or whatever it is called in the language on your macbook). Or you might have to press cmd+f first.
- Navigate the Accessibility preference pane by shifting focus around to different tabs, buttons etc with combinations of tab and arrow keys.
- When a checkbox is in focus, press space to check it.
- The checkbox you are looking for is called something similar to Disable trackpad when external mouse is connected in the mouse and trackpad tab.