The ability to RED Pill and install DEB files has been withdrawn from the new N900 Firmware update v1.2009.44-1, oh boo hoo, sob sob!
But wait, there is a way!
1) To install you need to transfer the downloaded *.deb file to your N900(for example Documents folder)
2) Open xterm and type(you need rootsh for this):
dpkg -i MyDocs/.documents/*.deb
3) Close xterm, go to More… and your application should be there.
NOTE: You may need to reboot your phone after the installation.