There are a few ways to log into WHM. The best way depends on if your domain name has propagated yet.
If you recently purchased a new account with Microhosting or have run out of bandwidth, you will type the following information into the address bar of your browser:
http://yourIPaddress/whm
(Example: http://10.20.30.40/whm)
If the domain name is already propagated, then you can use the domain name to log into WHM. Here is an example:
http://www.yourdomainname.com/whm
or
http://whm.yourdomainname.com/
Use your domain name instead of yourdomainname.com used in the example.
Why does one work and the others do not?
There are a couple reasons for that. If the versions with your domain do not work, and the IP version does, then your domain has probably not propagated yet. If you can access it at whm.yourdomain.com but not the others, then the port for WHM is blocked by your ISP, router or company (if accessing the internet from your corporate offices). Accessing WHM via whm.yourdomain.com forces it to use port 80, which is the same port regular web pages use.