1. download ARMEL binary diff for ARMEL version, for exam - http://www.filewatcher.com/m/diff_2....86658.0.0.html

2. extract files from it by
dpkg -x diff_2.8.1-12_armel.deb

3. copy 'diff' binary to your preferable directory - /usr/bin or /home/user/bin (latest requires an update to PATH environment variable).

Unfortunately, you can't install 'diff' package on Maemo5 because it contradicts with busybox - it also has 'cmp' commands which is included in busybox.

BTW, package also has locale messages file and you can move it to right place too (see unpacked directory structure for an exact location) but 'diff' works w/out it.