WordPress has different requirements than the traditional methods of building websites with HTML and CSS, like the MobileMe hosting service and the iWeb design and authoring software.
WordPress needs what in the web hosting business are commonly called “server side includes.” Those are languages or other kinds of software that WordPress needs to run. They’re called “server side” because they run on the server. In contrast, HTML and CSS run in the web browser on the user’s PC or Mac and don’t really need anything special on the web server.
In the case of WordPress, you need PHP, a programming language, and MySQL, a database server. Those aren’t available at MobileMe, mostly for reasons of simplicity of the service. And you can use PHP with iWeb, but it’s complex.
So you can keep your site on MobileMe and use iWeb with it, but if you want to learn and use WordPress, you’ll need to get an account at a web hosting service that supports PHP and MySQL, which is almost all of them.
Try GoDaddy or BlueHost or check WordPress’ own requirements and commendations for web hosts: Hosting WordPress « WordPress Codex.
You can get WordPress-compatible hosting for $6-8 a month, and many of those web hosts offer one click installs of WordPress for you.