We saw BBM land in the Windows Phone Store recently, but users were unable to download the app as it required you to be enlisted in a special beta preview. Today, BlackBerry have launched the signup process to be entered into that beta, which will allow you to test out BBM for Windows Phone and report back on bugs.
Upon signing up, BlackBerry will be in contact shortly to confirm your position in the beta. Once that's done, you should be able to download BBM from the store, and begin messaging your BBM friends. As this is a beta, you're going to have to report on bugs and give feedback to BlackBerry.
To participate in the beta, you're going to need a BlackBerry ID along with a BlackBerry Beta Zone account. Once you've got all that, just sign up for the Windows Phone BBM Beta by entering in a few details about your Windows Phone. Once that's done, you just have to wait for BlackBerry to be in contact.
Visit the link below to signup for the BBM for Windows Phone beta.