How do I do this...

I need to have my domain, specialneedsclinic,net, direct people to my root directory (which it already does), but...

I also need to have my other domain, specialneedswiki.net, direct people to a sub-directory "/wiki"

How do I set this up?  I've already purchased both domains and I've "parked" them within my main account "chesapsych.com"

Thanks
Eric

Since you have a primary domain "chesapsych.com", both the other domains should be "add-on domains", which gives them their own private areas. When adding them, the default directories would be .../public_html/specialneedsclinic/ and .../public_html/specialneedswiki/. You can give them shorter names if you want, e.g., .../public_html/clinic/ and .../public_html/wiki/ -- the public won't see those names. You will end up with three completely separate websites sharing your account. This assumes that the three will have different content, and you're not just using one name as an alias for another. In that case, you will have to worry about search engines dinging you for "duplicate content", so I would avoid that if possible.