Add programs from the network

You have to Publish applications through Group Policy. If you do that, then they will be visible in this box.
If you are not running Active Directory with Published applications, then you can't add anything to this box.
Now, if you just have a program copied to a network share, and you want to install from that, the only way you could get to it from the Add/Remove Programs control panel applet is to choose the CD/Floppy option. Let it scan for a cd and a floppy. If there are none in the workstation (and for this to work make sure none are in the drives), you then get a Browse button you can click to navigate to where the install program for the program is. At this point, you could navigate to the network share where the program you wantto install is located. Select the setup program for it (or install program, or MSI program if it has one) and you are off and running!

Just to elaborate a little on this, you need to create a group policy object that applies to the container that the user object resides within. Under the User Configuration/Software Settings area of the GPO, add the MSI for the software package you wish to publish.

There are several ways you can deploy the application, but the method that you seem to be looking for is "published". This makes it visible in the control panel under Network Apps.

If you assign the package to the machine (rather than the user) it will install upon reboot. If you assign the app to the user (instead of publish), it will appear in the Start Menu as though it is already installed. When a user selects the program, it will complete the installation.

Hope this helps.