I cann't get to buy some keywords that are very important to my business. I can not buy them because of the trademark rules but I have seen that one advertiser is buying it. Do you know how? Thanks
They would have to get permission from the trademark holder, then show that to google to get an exception. Otherwise they risk trademark infringement, getting suspended, and/or cancelled.
It's worth checking the Google trademarks as you may be ok if your ads are appearing for broad match, non trademarked keywords. Check out the Nike shoes example given here: http://www.google.com/tm_complaint_adwords.html