You can definately sign up if your not in the US, I am in Canada and I run ads through the US adCenter. The only difference is that the prices are in US and if you don't target your country or area, you won't be able to see your ads. For example, if you are in australia and you only target the US, you have to search from US to see your ad, or use remote search from a US computer. I think it's definately better than adsense, check out their information site http://advertising.msn.com/microsoft-adcenter