I installed the pre-modded phpbb SEO (latest version)

However, I'm not sure how to configure it properly.

When I go to one of my forums for example, it gives a generic link: http://cheftomcooks.com/forum/forum76/

How can I replace "forum76" with the actual Forum category name ?

And then for posts: http://cheftomcooks.com/forum/topic12357.html

How can I replace "topic12357" with the actual title of the topic ?

You should read around a little before you install:
Read this
phpBB SEO Ultimate SEO URL Presentation

As you can see there are different rewrite modes with a lot of configurable options, so you should really read the whole thing and decide on every option what suits you the best.

And having the topics called topic12357.html is not actually bad. It is the simple mode, and it's really powerful without difficult configurations, so if you don't want to dig into the problem, it's not bad to just leave it this way. It is still a big leap from normal phpBB.
viewtopic.php?t=276 and topic276.html seems no real difference for a human obsever, but it makes all the difference for the crawler robots.

Yes, especially since with SQL rewriting you can still set custom urls with keywords for important topics on a per case basis. It's the setting used on phpBB SEO (virtual folders in mixed mode with sql rewriting).

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