Do you know about tab-completion in Sage? Once you define a poset p, type p. and then hit the TAB key. It will list all of the possible methods attached to p. One of those options is p.incidence_algebra. Typing p.incidence_algebra? provides the information that it takes one argument, the base ring. So p.incidence_algebra(QQ) may be what you want.
sage: search_src('incidence algebra')