The modeler automatically generates a new unique admin password every time you start the runtime from the modeler.
To determine this password you can look at the environment variables of the runtime java process. Easiest is to use a tool like process explorer.
See image for an example of what to look for in process explorer. This blog post describes how you can use the monitoring api using some scripting on your local machine: http://www.mxblog.eu/link/post/a-script-to-set-the-lognode-levels-in-mendix
You get that with an invalid password. Keep in mind that the request header must have a base64 encoded password not the plain password.
So if the password is the nr: 1 your X-M2ee header should say MQ==