Okay in Ubuntu I would just open a xterm and type

So how do I do this on the N800, I've tried searching here and google, so its driving me nuts.

HEEEELLLLPPPPP!

/etc/apt/sources.list.d/hildon-application-manager.list

Please treat me like an idiot, what do I do with this?

Edit it?

(with your favourite text editor)

using emelFM2 to edit /etc/apt/sources.list.d/hildon-application-manager.list and I get permission denied?

Do it as root.

Okay, please I'm going round in circle and ready to throw my N800 at the wall.

Can you please do a step by step instructions for editing this file? What to open, what to type, etc..

sudo gainroot gets you a root shell; if it doesn't you need to either hook up to a Linux machine and use flasher-3.0 to enable R&D mode (which has other effects, too), or install the becomeroot package, which kills the test for R&D mode so gainroot works.

Once you've got a root shell, if you want it like Ubuntu's default, you need to use visudo to add a line with ALL, nopasswd, ALL....

EDIT: OK, here's the actual line to add:

I've got becomeroot already installed and I can sudo gainroot.

Its what do I type next? If I edit with emelFM2 I get permission denied.

Well, you can try using whatever editor you like; some GUI editors will have session-related issues from running root, and won't work; if you can't get your favorite, go with your favorite screen editor (I use JOE (with stylus support) for almost all editing on the N800), or maybe even sed...

(Sorry if the non-specificity is annoying, but while all the respondents so far are probably arrogant enough to tell you which editor is best , I think none are sufficiently presumptious to tell you which to use. If you want to use that piece-of-<insert rival editor>, I'm not gonna stop you...)