Method 1:
Open your [color=yellow:9d671677e5]UnrealTournament\System\User.ini[/color:9d671677e5] file in [color=Cyan:9d671677e5]NOTEPAD[/color:9d671677e5] or any other [color=red:9d671677e5]TEXT EDITOR[/color:9d671677e5] and add it to an unused key... mine's on my spacebar:
[color=red:9d671677e5][Engine.Input]
***skipping a bunch of lines that aren't relevant***
Pause=Pause
CapsLock=LookToggle
Escape=ShowMenu[/color:9d671677e5]
[color=green:9d671677e5]Space=MUTATE BDBMAPVOTE VOTEMENU[/color:9d671677e5][color=red:9d671677e5]
PageDown=LookDown
End=mutate asc#get#window[/color:9d671677e5]

See that line that starts with 'Space='?  After the = is 'MUTATE BDBMAPVOTE VOTEMENU'... that's the mapvote command.  Find an unused key and put it there. For example:
V=MUTATE BDBMAPVOTE VOTEMENU

Method 2:
When you see this screen come up, click on the [color=yellow:9d671677e5]Config[/color:9d671677e5] tab.
[img:9d671677e5]http://img136.imageshack.us/img136/3729/20080102shot0157lj0.jpg[/img:9d671677e5]<br>
Under the Config tab, click in the space where my screenshot says 'Space' and then press the key you want mapvote to be assigned to.
[img:9d671677e5]http://img136.imageshack.us/img136/5434/20080102shot0158ph1.jpg[/img:9d671677e5]

If neither of these methods work for you then maybe you should give up on computers and get yourself an Xbox or something.

Anyone wanting more info about mapvote can go here: http://forums.utassault.net/showthread.php?t=55286 or google it up for yourself http://www.google.com/search?source=ig&amp;hl=en&amp;rlz=&amp;q=mapvotela13&amp;btnG=Google+Search