The search function in the IRIS Forums has not been activated yet. If you would like to have it, please request it with a ServiceDesk ticket, using the link at the bottom of this page (Report an issue with IRIS).
Meanwhile, please note that the Forum pages are public and are therefore crawled by all major search engines, such as Bing and Google. It should be possible to conduct specific searches by using the appropriate keywords and also "IRIS stakeholder forums" in the search.