"Please refer to the advertisement" means to look at the advertisement for information, presumably about the product or service being offered. "Please refer the advertisement" means to tell someone else about the advertisement, as in "Please refer the advertisement to the newspaper", which means that you would like the newspaper to publish the advertisement.

Please refer to the advertisement is correct if you are asking someone to respond to the ad. The other please refer the advertisement could be correct if you are asking that the ad be referred to to something else. What I think you mean is for someone to use the ad to evaluate the product. If so, the #2 please refer to the ad is correct.