DirectoryPress.

If you are going to design and build this directory following these steps, this is where you are going to have to spend some money. It is not a lot of money for buying all the functionality and themes that comes within this theme. You will also need a domain name and web hosting. But that sadly is that. No expense spared here.

When finished with purchasing this, you will get the package in a download link, and quite thoughtfully, access to a client area where you can access the latest version of your products.
Access to the support forum is also conditional upon being a customer and having paid for the theme. Makes sense, no spammers and viagra sales.
The download is in a .zip file format.
You need to upload (ftp) either the zipped file, or the unzipped file to the location in your public_html folder where you have placed all your wordpress files.
Look for ../wp-content/themes/ and upload the content in here.
Go to your administration control panel when finished.
Activate the theme.

Activation screenshots

Not that complicated. In the administration panel under the themes header you would have activated the theme. When you try and access your template you will get the following screen.

When you have entered the serial key you received with your purchase the screen will change, and you will get access to the DirectoryPress control panel.

Uploading themes tutorial