Repository write access is handled by ssh tunnels that require the ssh public/private key system.

On Windows, You can use PuTTYgen to generate your public and private key pair.

On *nix (including Mac OS X), you can just pop open a terminal and use ssh-keygen.

As an alternative, less secure, method we also will generate ssh keys for you. This is accessible at: /users/generate-keys/

When you have your ssh keys created, upload your public key to /users/current-profile/public-key/edit/ and you should be able to use that key to have write access to projects that you are a member of.