Best way to get faction numbers is to target the mob and click "Target Info" in HB. That way you're sure you're getting the right number.
Information about Crypt Crawler:
Name = Crypt Crawler
Wowhead Id = 25227
Faction = 1988 [Undead, Scourge]
Location = <2440.934, 5487.333, 29.34689>
<Vendor Name="Crypt Crawler" Entry="25227" Type="Repair" X="2440.934" Y="5487.333" Z="29.34689" />
This is what HB output looks like;
Code:Information about Crypt Crawler: Name = Crypt Crawler Wowhead Id = 25227 Faction = 1988 [Undead, Scourge] Location = <2440.934, 5487.333, 29.34689> <Vendor Name="Crypt Crawler" Entry="25227" Type="Repair" X="2440.934" Y="5487.333" Z="29.34689" />
As you can see it gives the Wowhead Id for the mob but directly below is the Faction Id which is what you add to the Factions line in your profile.