Have a look at this:
http://en.wikipedia.org/wiki/Sitemaps
AdminCP -> Options -> XML Site-map
Set the information as required including a location where the site map can be written to your website. This location also needs to be accessible to the search engine bots. If your site is new, I wouldn't worry too much about changing the default settings until you understand how site maps work a little better.
So....
Enable: Yes
Automatic Site-map Generation: 14 Days is the default, I believe.
Default Priority: 0.5 (no reason to adjust this yet)
Automatic Site-map Sub-missing: Check the search engines you want.
Site-map-file Path: Needs to be a relative path in your web directory that is writeable for v Bulletin and readable by a web search engine.
Site-map Per Page: No reason to adjust if your site is new...
Save those options and then go to AdminCP -> XLM Sitemap -> Rebuild Site-map
Run it to rebuild to see if you get any errors. If it cannot write to the specified location, it will let you know.
Good Luck.