Finding a SIM card slot which fits straight onto the Aspire one is hard. The most commonly available is pinned out for the Eee PC. You have to reverse every pair of pins to make this compatible with the Aspire One.
Maybe one day the correct type of slot will turn up on eBay once there's enough of these in the market. I've kind of done this a bit messy with stranded wire because I couldn't find my prototyping wire at the time.
While you here, You're going to need to install a couple of links (circled) so that the 3G slot gets its 1v8 and 3v3 power rails. I had all these ideas about how I might put proper zero ohm links, SMD fuses, or even ferrites in here but in the end I caved and put in wire links as I didn't really have anything suitable on hand.