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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.

Alternatively to tab-completion, you can search the documentation (for example at http://doc.sagemath.org/html/en/reference/) to get http://doc.sagemath.org/html/en/reference/combinat/sage/combinat/posets/incidence_algebras.html?highlight=incidence%20algebra#module-sage.combinat.posets.incidence_algebras.

Or you can search from within Sage:

sage: search_src('incidence algebra')