I've already configured AMT on computers ranging from HP 7800 to 800 g1, in all of these after CTRL + P there is the "Intel AMT Configuration" option, but on the DC 5800 this option doesn't appear
in the attached image those are the options I have, but as can be seen in http://vpro.by/img/HP8510p-connecting-AMT-client.jpg there should be another option
also in the Computer Setup (F10) guide http://h20566.www2.hp.com/hpsc/doc/public/display?sp4ts.oid=3658081&docId=emr_na-c01363881&docLocale=en_US for this computer the only mention to MEBx/AMT is to enable/disable the CTRL+P visibility on boot, so nothing to touch there afaik