It's pretty simple! Click on the 'Search' key in the menu bar at the top of the page. For a simple search, just put the term you're looking for in the space available. The search engine will search all threads in all fora from any date for the term you nominated. This is a bit of a blunt instrument, but it can be useful. If you wish to be a bit more discriminating, click the 'Search' key as before, then click on 'Advanced Search'. Now you have the option to nominate which forum(s) you want to search in, whether you wish to search within threads or simply by thread title, what date range, and even by member's name.
Scroggs