If you are using firmware 1.2009.44.1 this Red Pill mode on not available any more. For more details read this.
Here is small tutorial how to install Debian files, .deb, in your Nokia N900. Actually is very easy to do, follow the steps.
1. Go to Applications Manager and then click on Applications catalogues.
2. Click on create a new catalog.
3. On Web address field type Matrix. Don’t click Save, click exit.
4. Now you can see the questions Which pill? Press the Red pill.
5. If all is OK you will have new field Install from file in Applications catalog menu.
Please note this in not recommended to do and do this only if you receive .deb files from sources you trust. MaemoCommunity.com is not responsible for damage that my occur.
And here is video showing how to install Debian files.