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On the script's download page there are some instructions (though they could probably be better). Here's a good video walkthrough. To grab hits quickly you'll have to set your timers low. If you have too many hit watchers going to quickly you will start to get page request errors from mturk so watch out for that.
To grab a bunch you have to go to the HIT and click the "Watch this HIT" button. From there make sure "Auto-accept" is checked and "Stop on accept" is NOT checked. It'll keep attempting to grab those hits until you tell turkmaster to stop.
Yeah 25 is the most you can have at a time, but if you can do them quickly enough it'll keep grabbing new ones while you do the ones in your queue.
It's useful to use the mmmturkeybacon Queue Order Fix script while doing this.