First, you need to ensure that their power rating is as high, or higher, than the power of the amplifier. (If you connect 5W speakers to a 50W amplifier, you'll 'blow' the speakers, rendering them unusable, as soon as you turn up the volume). Most computer speakers can't handle the power from anything other than the smallest amplifiers.
Second, you need to ensure that the impedance of the speakers (roughly) matches the output impedance of the amplifier. So you'd be looking for speakers with an impedance around 5 to 10 ohms for the amplifier I've linked to above. Something like these would do the job:
http://www.richersounds.com/showproduct.php?cd a=showproduct&pid=JBL-SAT300
However, I've been looking at the specification of your Creature II speakers. The all-important word 'active' is there, which means that they're not just speakers. They've also got their own amplifier built in so, theoretically at least, you can use them to accept the output from your mixer. You won't get much in the way of volume because the main speakers are rated at just 8W each. The results would be far from brilliant, especially if your mixer deck is 'passive' (i.e. it doesn't incorporate a built-in pre-amplifier). This is because the signal coming from the cartridge is very small compared with the power that comes from the headphone socket of an iPod or the speaker socket of your PC.
You can certainly give your Creature II active speakers a try but don't expect any great results. For far better results, invest in a decent amplifier and speakers. (If you check your local free-ads paper, you should be able to pick up both fairly cheaply).
Chris