Creating a simple e-book with MobiPocket Creator is relatively easy. The steps for doing this are as follows:
Grab the latest version from here: http://www.mobipocket.com/en/downloadsoft/DownloadCreator.asp
Complete the installation, paying particular attention to the page below where you are asked which edition you would like to use:

You will now have an icon on your desktop that you can use to launch MobiPocket Creator:

The start page looks something like this:

Under "Create New Publication", click "Blank publication". You will be presented with a wizard that asks you a few questions about your publication, including:
- the name
- location for storing the project files
- character encoding (changing this to UTF-8 is highly recommended)
After completion, you can begin adding content to your book by dragging-and-dropping files into the window:

For example, you could create an HTML file like so:
<!DOCTYPE html>
<html>
<body>
<h2>The Beginning</h1>
<p>This paragraph introduces the very beginning of your book.</p>
</body>
</html>
...and then drag it into the window. It will be copied to your project folder.
Click the "Metadata" link on the left of the window to edit the title, author, etc.
When you are ready, click the Build button in the upper-right corner. Leave the settings at their defaults and click the Build button:

In my case, the tool produced a file with a .PRC extension. You can simply rename the file to .MOBI if you wish.
Important Note
MobiPocket Creator is licensed exclusively for non-commercial use only so you will need to get in touch with them if you want to actually publish content with it.