In one of the pictures from this thread:
It is the brown socket just to the left of the speaker...It appears to be wired to the battery clips on the other half of the radio.
jr
PS: Found a nice youtube video (by batterymaker) of one loaded with "D" cells and battery connector plugged into outlet left of loudspeaker...caption on video says that a 2761 battery can also be used:
http://www.youtube.com/watch?v=K--c6jKU4qo
jr