Sunrise missing from wp-config.php

As you can probably tell from the Topic, I'm a php newbie but enjoy learning :slight_smile:
I cant uncomment the Sunrise line in wp-config.php because it's not there. I suspect the person who set up some redirection ages ago (which I don't really need any longer) deleted the line. Or else it didn't get put in by the domain-mapping plugin.
Can I just go ahead and add it - does it go somewhere special? Are there any other lines (eg ones that may indicate redirection) that I should remove?

I've doubled checked and sunrise.php is in wp-content and domain-mapping.php is in mu-plugins. But when I open wp-config.php there just isn't anything about vhosts or sunrise or anything.

Currently the mapped domain goes into a loop (new domain, primary domain signup/login, primary domain home page, then back to the new domain again).
Thank you in advance for your kind help
PS Yes, I did so a search but didn't see anything under "missing Sunrise" :stuck_out_tongue:
PPS I've successfully set up exactly the same wordpress 3.0/buddypress install on the same server in another subfolder and domain mapping worked, so there shouldn't be anything different with this one except it's older and had redirection set up.