There should be an editing toolbar above where you type in your post. The left button labeled ‘b’ starts bold, so put your cursor where you want the bold part to start, then click the button. When I did it, I got the tag <strong> rather than <b> for bold.
I looked this up on the bbPress support forum, and bbPress has a restricted set of allowed html tags for anybody other than admins for security reasons. Surprisingly, <b> was not on that list.