On desktop keyboards the numeric keypad is usually separated from the character set, but laptops don't have a separated numeric keypad. There are different ways to emulate one, e.g. with the Fn key or with NUM-LOCK key. Also external numeric keyboards which connect to the PS/2 port (or USB, RS232) are available.
As described above, the numeric keyboard has to be used if you want to change the X11 resolution by typing <CTL><ALT><+> or <CTL><ALT><->. If this doesn't work or is too complicated, you may use gvidm Running gvidm will pop up a list of available modes and allows the user to select one if desired. This makes it perfect for running from an application menu or a hotkey, so you don't have to use ram for an applet constantly running. If you are running dual or multi-head displays, it will give you a list of screens so you can select the appropriate one. Also you may use xvidtune [-next | -prev ]. To check the current resolution you may use xwininfo -root, if xvidtune is not at hand.