I was struggling with this so I will post what I found out about XPAT and how it works in altbinz:
XPAT searches the server for articles matching the search term. You can search by subject or poster (author).
Note that to select the newsgroups you first need to download the newsgroups using the Newsgroups bar, download the list of news groups on your server, then select the groups you want to search on. Then go to the XPAT search bar, and click on the "Group" box (on the left side) which will reveal the groups you selected in the newsgroups bar. Select the newsgroups you want to search, then do your XPAT search based on poster name or subject.
You can limit the search by message number or by a specified number of posts back from the highest current post on the server. For example, if you want to search the last 5,000 posts, check the "Last" box and enter 5000 in the adjacent window (to the right).