The practical way to install software on Openmoko is with the Ipkg package manager, possibly via the graphical Application Manager. Naturally, you need to have obtained an Openmoko system by other means initially.
In the shell, "opkg update" will update the list of available packages, "opkg upgrade" will download and install all packages that have a newer version available and "opkg install new-app" will download and install "new-app".
Have a look at Users Repositories to get addresses of alternative software repositories, e.g. ScaredyCat's ipkg repository.