If a client has registered a domain name on their GoDaddy account, how would I make it point to a directory on MY GoDaddy account?

Do you mean YOUR HOSTING account ?
That would be a DNS issue.

You take YOUR nameservers and plug them into your friends domain. Next go to your control panel, and add the domain in question as an "add-on domain". Upon doing so, you'll be prompted for the "directory" that will act as "root" for that specific domain. Set the directory name you were speaking of earlier.

This might help: http://help.godaddy.com/article/4688

Are you looking to have redirect from their domain name (web site) to yours?
Just can't understand what you meant

how would I make it point to a directory on MY GoDaddy account

On your web hosting account just add domain name your customer registered - and then using your web hosting upload content.

its confusing as to what you are asking. you need the domain to be pointed to your godaddy account as in you want the domian under your godaddy account? then there is an option to transfer the domain (street name free push) from one godaddy account to another. but if you want your client to own the domain but you want to host it under your hosting account, then your client needs to change the default name server to custom name server with your DNS info. you then setup the addon hosting by creating a folder under your hosting account and direct that you that specific domain. you can do that by going to yuour hosting account, then domain management

Thanks!