I need weatherproofing/sealing advice.

I am replacing a plank in an exterior stariway landing. It will abut
the outside stucco wall of the house (Los Angeles). The old planks
have some kind of pretty heavy duty stuff that keeps water out where
the plank meets the wall. There are photos of it he

http://members.dslextreme.com/~dmorg...nkreplacement/

What product should I use and how is it applied, to properly seal up
the new plank at the wall?