A couple of years ago I posted looking for information on light sensors that could be effectively used with the MSC-400, with the voltage sensing accessory as an intermediary connection. While the discussion was fruitful on other subjects, nothing was ever recommended for light sensing.
I've been meaning to post back for a while with some links to suitable products, so here goes. When searching the web it all comes down to the selection of terms.
A quick search on eBay for "dusk switch" seems to bring back some nice products that would readily connect to the MSC-400 using its own 12v output and the voltage sensing accessory.
Here are a couple, though these links will disappear eventually.
$13 shipped: [Link: ebay.com]
$37 shipped panel-mount style: [Link: ebay.com]
These come with bare wires. There's also the Xantech SMLIT01 which is generally used for monitoring LED or other equipment light but should also work for ambient or outside light level if you stick the sensor on a window facing out. The stereo mini plug will have to be cut or a connector to bare wire supplied to then connect it back to the MSC-400's 12v supply and voltage sensor.