<!-- google_ad_section_start --> <!-- google_ad_section_end --> <!-- google_ad_section_start(weight=ignore) --> Here the complete info https://www.google.com/adsense/support/bin/answer.py?hl=en&answer=23168
be sure that the specific text is related to the content of your page. Otherwise you could be banned from adsense.
you need section targeting. you can find more info. about section targeting on official adsense help site