My magic mouse broke , how do i pair my new one????

You might want to have a wired mouse ready just in case - here are instructions (and they do mention that you need to have a wired mouse/keyboard, but that it may be possible to pair without it):

http://support.apple.com/kb/ht2845

Install the batteries, turn the mouse on. If your mouse isn’t paired with a Mac, the light blinks, indicating your mouse is in discovery mode and ready to pair. In other words, the mouse will initiate the pairing process, do you don't need another pointing device to navigate to the Bluethooth settings.

I'm not sure this is technically necessary....but I've had some tricky moments with bluetooth devices on my iMac when I first got it. If you have a USB mouse handy you can go in and delete the profile for your old magic mouse (minus sign) and then install your new magic mouse. I had to do this once but luckily I had a magic trackpad installed too which made it easy to delete the old profile. There may be a way to use your keyboard too, but I'm sort of a pointer/trackpad user on desktops.

Why would you think you need a wired mouse to install a wireless mouse? Did you need a wired mouse in order to set up the last magic mouse you had?