- You rename folder to 'admin', or whatever you want; change in web.config some lines - where is umbraco -> admin. Choosing this solution might not be that nice. When you will install a new package that will try to install on /Umbraco folder, so you will have to move files. Beside you still have to make some updates in Css files as it is hard-coded the paths.
- The second solution it is a very simple one :) Go to yoursite/Umbraco/UrlRewriting.config and add this line in <rewrites>
Good luck, ;)