Did you already download the .deb onto the tablet? If you can find it in the file manager, you can probably click on it there to run the installer.
To use dpkg -i you have to be in the directory where you have the .deb file or specify the complete path, but unfortunately I'm not sure where the n900 stores downloads by default (I'm still waiting on Dell to ship me mine). If it stored in /home/user, you could type 'sudo dpkg -i /home/user/filename.deb', for example (I'm assuming you need sudo to install, but could be mistaken). Googling dpkg should turn up any syntax help that you need, but without knowing where you downloaded the package I can't be of much more help. :/