To search through a particular column other than the first, use the following syntax:
| nismatch column=string tablename nisgrep column=reg-expression tablename | 
This example searches through the depts table for all the entries whose second column has a value of SanFran:
| rootmaster% nismatch -h Site=SanFran depts.doc.com rootmaster% nisgrep -h Site=SanFran depts.doc.com |