Second you must type in /dynmap webregister IN GAME
Third you must go to http://www.minecraft.xandorus.com:8123/
It will redirect you to a login/registration page
Below the login details it will have an option for you to register.
Type in your MINECRAFT username and ANY password (Note DO NOT use the same as your minecraft password!)
Now referring back to where you typed in /dynmap webregister, it will have given you a code in the format of xxxx-xxxx
Either minimize minecraft and your browser and put them side by side in order to copy it, or write it down in notepad and paste it in to the box saying
Click register.Registration Code
You are now registered for dynmap!