Newcomer to WPMU – Can't edit themes?

Hello all. I decided to take the plunge and install a WPMU site. I develop WP themes for clients and usually will make a test site on a subdomain on my VPS so the client can view it, play with it, etc… before the site is done and goes live. Now I have a lot of subdomains with multiple logins etc…I thought combining those sites in WPMU would give me the ability to maintain them better and keep them under one login.

I just finished installing WPMU tonight using the subdomain feature. I made a site and went to start editing a child theme and I can’t find anywhere to FTP into the site to edit the theme. Should I have used the subfolder instead of the subdomain option? Or am I just missing something?

I can use the theme editor but that’s not efficient in my workflow.

Any advice would be appreciated!