Try recording it, this way:
In versions of Word prior to Word 2007:
1. Click the Tools->Macro->Record New Macro menu item.
2. Under Assign New macro, click the Keyboard button.
3. Enter the key you want to assign the macro to in the Press New Shortcut Key text box. Note that CTRL and ALT keys should be pressed with the actual key to register.
4. If anything comes up in the Current Keys list box, consider a different shortcut -- this one is already assigned.
5. When you're satisfied, click the Assign button, then the Close button.
6. Enter your keystroke sequence. Note that the little Record Macro pop-up menu has a standard recorder pause button. Handy if you have to try something or use Help to look something up in the middle of recording the sequence.
7. When you're done, press the stop button (the usual square recorder stop icon) on the little Macro menu.
In Word 2007:
1. Press ALT/T, then M, then R (the same as above!).
2. Continue with Step 2, above.
Hope that helps.