You use your mouse to move over either the start or achievements button and click on it.
Hey if you want to remake it and improve it either by fixing the problom with touching black on the start menu sending you to achievements or making it smaller you are welcome to and will get credit for improving it. This was all done by me I always wanted to have a menu with more then 1 button but due to the limit of 1 script and 1 sprite it took a while. Basically your mouse point has a small dot on the end of it that acts as a sprite and the background has a bunch of different colors that are used for menus.