1. Install rootsh from Application Manager
2. Open X Terminal
3. Type: sudo gainroot
4. Type: apt-get install gstreamer-tools
5. Confirmation to install gstreamer-tools will be asked: enter yes
6. Wait for installation to complete. Do not close X Terminal
7. Type: exit
8. Type: gst-launch v4l2src device=/dev/video1 ! xvimagesink
9. Behold!

Explanation
- After typing a line, in X Terminal, you must press enter key to actually issue the command
- At step 3, you become root, the system administrator that can do literally anything on a Linux machine, included breaking everything. Therefore, never become root without a very valid reason; when you do have one, quit being root as soon as possible.
- At step 7, you quit becoming root, for the sake of the above wise advice.
- At step 8, you create a gstreamer multimedia pipeline. The imput is a Video4Linux device, identified by the path /dev/video1: this is the N900 front camera. The output is xvimagesink, which basically says to show output on screen.

Sorry if I am saying things in a very pedantic way. I do not know your level of Linux expertise.