Here's how to do a basic search as well as an advanced search if you want to limit your search to certain boards or limit the amount of results you have to wade through.
Note: These directions are for searching on a computer. See the note at the bottom for searching on a handheld device.
Basic Search: Fill out the search box in the upper right corner of the page. Put quotes around specific terms if that will help narrow your search. Press Enter. That will usually bring up a whole bunch of results.
Advanced Search: First follow the Basic Search procedure. After the results page loads, click on the Search box again (I know. NOT intuitive!) A drop-down menu will offer you the choice of searching Forum (meaning, Blueboard posts) or Members (looking for posts by a specific member's screen name). Click Advanced.
Now you're able to set your search parameters. You can have results sorted by date, by relevance, and by the whole Blueboard or just certain sub-boards. Whe you don't want to have to sift through results from boards you're not interested in, Uncheck "Search All" in the lower left corner (on a laptop -- the checkbox might be partly hidden by the blue Search button) and then click on the ^ above the box to see a list of all the boards to choose search in. Check the ones you want to include and [this is important],
scroll all the way to the bottom of the page where the blue Search button has been bumped by the list of boards. Click on the blue button.
Voila!

You are now able to find that half-remembered discussion that was a tangent hiding in another thread.
TABLET AND PHONE USERS: Search box location may vary on tablets and phones, depending on screen size, but should be visible without scrolling. On my phone (6" screen) the search box is at the bottom of the first page. Pressing the black gear to the right of the gray Search button reveals the Advanced Search function, but I have to scroll the page up to see it.
FWIW, it's a real pain to search on a smaller screen. Use a

if you possibly can!