Thanks. What about now?

5:37 pm ?
why ?

Thanks. It's just that my task bar has minimised so I can't see my clock. And the one under the television is behind me.

The .deb files are packages for Debian based linux systems like the Nokia N900. They are compiled for quite some computing architectures, one of them is armel. You will only install packages for the armel architecture on your N900. The application to install .deb packages is called dpkg.
Sometimes the postinstall configuration of a .deb is not possible because of missing dependencies.
Most of the time the more advanced apt-get is able to install the missing packages.
One prerequisite to work with dpkg and apt-get in X-Terminal is a working root login. For this to work you should have rootsh from extras repository installed. Then will install all of the .deb packages that are contained in the folder PathToYourPackageFolderHere.
Any questions? Feel free to ask here!

ammm actually i am a newbie to n900 , dont have any knowledge on commands an x-terminal , need more explanations :$
or you can say , i need some easy words :P

Hey, this could really be a long, long story, but i try to make it short, having my attention to some family responsibilities on a sunday afternoon in germany too.

Linux is the base of the Nokia N900. It is a system which makes it possible to use your phone work like a computer!!! On the linux base maemo is running. It provides the layer for the applications, like calendar, email, browser and all the others.
One of the applications is X-Terminal, a terminal emulator, that can execute linux commands. But not all of them, and not with all of the options of a Linux PC, because it uses just busybox, a stripped down linux shell for small devices. To be able to use X-Terminal for all the commands you need to install an application in your Application Manager called rootsh. rootsh enables you to run commands that the default user is not able to launch.

The actual package management is made by dpkg [i think of it as debian package ...], it has many options, but really little artificial intelligence, so in comes apt-get[advance package ...]with some internal AI mechanics to correct errors and suggest possible solutions.

So start to your Application Manager, click on the upper bar, select catalogs or repositories, scroll down to extras repository and make sure it is enabled (mark the clickbox). Then after refreshing the package list search for rootsh and install it. Leave Application Manager and start X-Terminal.
is the command to get the rights for root[Linux Admin Account].
is the command to refresh the package list from the repositories.It is faster this way than trying the same from Application Manager.
is the command to install all the new versions for the installed applications. Always read the output carefully, better double-check nothing important will get removed! Never use this command, if you have extras-testing and/or extras-devel repositories enabled, because some strange errors might happen.
is the command to install a downloaded, single debian package for the armel architecture. It is crucial for this command to work, that you either are in the directory where you have downloaded the .deb file or give the complete path to the .deb file.
As mentioned in the thread missing dependencies [something like shared libraries] may be corrected by
The learning curve for succeeding in a strange environment ,like linux, is steep, so don't hesitate to ask, to search the forum for errors others made and others solved, or start threads in the forum.

greetz Micha