• iTunes (I’ve tested with 10.2.2 which is the latest version by the writing of this)
• Quicksilver

and some tuning =)
I assume you know or will learn how to use the basics of quicksilver before proceeding, there are tons of Quicksilver basic guides out there.
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On Quicksilver, go to its Preferences and under the menu Plug-ins you have to install (just click “All plug-ins” on the left tab):

• iTunes Modules
• Extra Scripts
• Hotkey Triggers
• User Interface Access (note that for this work properly Access for assistive devices must be enabled in Universal Access pane of System Preferences)

(I’m not sure if you need all of those, but it’s not gonna hurt having these plugins =) )
Then, again on the top menu, head to Preferences, then on the left tab, ‘Actions’, browse ‘All Actions’, you have to enable (click checkbox) “Play next in party Shuffle”.

Again on the left tab, click ‘Application’ and check ‘Enable advanced features’

Relaunch Quicksilver
Go to ‘Catalog’ and select ‘Quicksilver’ on the left tab, you must have ‘Proxy Objects’ checked on

Now you have everything to create your trigger, go to ‘Triggers’ on the top bar, and create a custom trigger (click on the + at the bottom and select ‘Hotkey’)
You will now record the action you want quicksilver to perform when you use the hotkey, so the action will be:
‘Current Selection’ (start typing it and find it in the drop down menu)
press ‘tab’, this will select the next box
on it will have to add the action
‘Play next in Party Shuffle’
Hit return or click ‘save’
Now you have to edit the trigger and add the hot key to it, select the recently created trigger. To show the info bar, by clicking on the small ‘i’ on the lower right corner of the screen. On the info bar under the tab ‘Settings’ look for ‘Hot key’ and click on the ‘edit’ button and select a combination of keys (command and some keys). Now go to the tab ‘Scope’ select ‘Enabled in selected applications’ and add ‘iTunes.app’ to the list.

Done!
Go to iTunes, select a song and test… it will be added as the next song in the iTunes Dj playlist. If the Dj is not playing and you start it, the first song will be the last chosen itunes DJ song.