@Kymera76 dpkg -i is an install command for files on the internal N900 memory, so if you havn't downloaded the file it wont exist and hence it wont install... So you can enable and disable devel forever without any result... If you have downloaded the file you just need to 'cd' to the directory and then do the dpkg -i command...

If this makes no sense to you then like MuhammadAG said this is not for you yet