hm, make sure the nslu2 can access the internet.
then telnet into it and chdir to /tmp.
You can copy/paste with the right mousekey, in all half-decent console apps.
wget will download a file from the given server and store it in the current directory. After the d/l has finished, you can just do the
ipkg install mt-daapd_svn-1372-1_armeb.ipk
the above example should work for the latest nightly files, filenames are subject to change 😉