Use the following rule to match on User-Agent String.

User-Agent String: Content Match

Rule type: Content Matching

Match Type: Regular Expression

Header Field: User-Agent

Match String: Mozilla/5.0 (Windows; U; MSIE 9.0; Windows NT 9.0; en-US)

You can view examples of User-Agent Strings at the following link: http://www.useragentstring.com/pages/useragentstring.php