Notices

after I downloaded by PC from site repository.maemo.org, I saved it on my PC..

then, how can I install the software manually?

anyone can explain this step by step???

Thankz a lot before..

If you need to install applications from maemo.org repositories then please use N900s own application manager. If you choose to install from devel or testing repositories then I suggest to really start learning how to google before doing any harm to your device

http://wiki.maemo.org/Extras-testing
http://wiki.maemo.org/Extras-devel

.edit
This stuff you can install from default application manager
http://maemo.org/downloads/Maemo5/

Or You can simply transfer the deb file of the software you want to install in the N900 masss memory and just open it with your File Manager!!!

edit: and you follow the on-screen instructions!! lol

What Boemien said will work, but if you are getting files from the maemo repository you should use the installer to download and install them. Just go to the "App. Manager" and find the stuff you want to install and follow the onscreen instructions. App Manager is a little slow, but it works through the linux package management that is built into the phone's operating system--the advantage of doing this is that if any updates come out, it will show you an alert and let you update the software. Also, working through the App Manager helps ensure (but, of course, not guarantee) that software will coexist peacefully and not overwrite each other's files on installation.

Think of the App. Manager as a combination of an app store and the "install/remove programs" tool on windows. And it works perfectly fine over wifi, so if data plan usage is the reason you are downloading via pc, wifi will help you avoid it.