Here’s what ASO’s tech support instructed me to do, which worked:
Hostname: WP had inserted “http”, but this had to be my website address
Username & Password: replace what WP filled in with cPanel login information.
WP is asking for this everytime I install a plugin now. Since it doesn’t seem to be happening for everyone, including everyone whose blogs are hosted by A Small Orange, I’m guessing there is a problem somewhere. Since this is not a shared computer, I allowed my browser to save the username and password information, though WordPress may do that on its own.
Hope this helps.