Basics
Pressing ALT + X on your keyboard will open a small box in the right corner of the status bar. Use it to enter the NumExt commands.
Since version 0.6 a NumExt clickable label has been inserted under the Tools menu of the main menu. In case Alt + X is used by another addon or menu, you can set up a shortcut key by typing (execkey z) for "Alt + z". The clickable "NumExt (exec)" enables the command box as in the shortcut key.
Windows only
In addition to the standard CTRL + [number] key combinations, you can use the following ones:
ALT + 1 will jump to tab number 9;
ALT + 2 will jump to tab number 10;
ALT + 9 will jump to tab number 17.
The ALT + [number] shortcuts were created in order to overcome Firefox's limits of AccessKeys Control to tabs 1 to 8.