Here's how I get it.

On your n800 (if it's running OS2008, if not you'd have to switch the year box on the webpage your about to go to for OS2007).

Go here. http://maemo.org/downloads/product/OS2008/pidgin/

Click the green arrow that says click to install.

Let it download and open the application manager (should open it by itself).

When it prompts you if it has permission through a yes/no/(and some other option I forget) button screen, click yes. Then wait for it to do what it wants, then it should prompt you again. Click yes again. Then it should be installed.