that is very good sign not to do it unless you are ready to bear the risk that your device becomes a brick (and after that it is necessary that you have the skills to reflash it. NSU @ windows could be incapable to do it, so: http://wiki.maemo.org/Flasher )

but after this intro:

open terminal (ctrl + shift + x)
write
find / -name appinstaller.deb
press enter
this should return something like
/home/user/MyDocs/appinstaller.deb

then write exactly the line find returned behind dpkg -i, like this:
dpkg -i /home/user/MyDocs/appinstaller.deb

and in the future: RTFM

http://www.lmgtfy.com/?q=linux+terminal+howto
http://tldp.org/HOWTO/Text-Terminal-HOWTO.html

youre requesting total spoonfeeding.

if you aren't a linux geek, you just have to wait for stable apps. the road to terminal commands isn't so rosy, it could render your device easily useless. and installing unstable apps can do almost anything to your device (delete everything ever existed in your device for example), without QA no-one will be responsible for anything. that is good reason imo to stick with extras. the apps will go fairly quickly to extras in the end so couple weeks waiting could save ones life when unstable app creates a worst case scenario.