I created a group three days ago. It is a private group (meaning people need permission to become members) but everyone can ask to join. I specifically marked "List this group in the directory", and yet I can't find my group in a Google search.
The Google Groups directory is a separate thing from the regular Google Search. The directory is simply the listing of groups available from the Groups home page (see bullet point #9 here).
As for why your group isn't showing in Google search, it's probably just a matter of being patient:
Crawling and indexing are processes which can take some time and which rely on many factors. In general, we cannot make predictions or guarantees about when or if your URLs will be crawled or indexed.
Another option is to use the Google submit URL. Don't spam it, but add that page, it has always worked well for me. I think you might be limited to 30 or so submissions per day, so if I knew that I needed Google to crawl it, I would submit before heading to bed, and when I wake up, it is always in the index. Relying on Google to auto-index you, especially for something new, so be proactive and just ask Google to crawl.