That's possible only if you have configured TP to copy the desired IPTC values from the image files into user fields in its database.
For this to work you must have "Automatically populate user fields from image metadata" on in Options | Prefs | Metadata, and you must have made one or more user fields (Database | Setup User Fields) whose names are identical to those of the IPTC fields that you want to copy into the database. The user fields must be of type 'Text' and long enough to hold the data. From then on, when you make (or re-make) thumbnails, ThumbsPlus will copy the values from the IPTC fields to the matching user fields in your database, which enables you to search them (ThumbsPlus doesn't search files, it only searches its own database, which is much faster and which allows you to search media items stored offline).
P.S. for a list of iptc / user field names, see
http://forum.thumbsplus.com/forum/index.php?topic=5916.msg5958#msg5958