Ah you can even still get them on Amazon!

http://www.amazon.com/Radica-IM-Wireless-Handheld-Device/dp/B000NX1LUM/

Best review:

Mattel's servers shut down long ago - I guess it was a nice idea, but not sufficiently profitable for them to keep the system going, So - DO NOT BUY THIS AS A MESSAGING SYSTEM FOR YOUR KIDS - for that purpose it's as useless as a paperweight! It contains the coveted Texas Instruments C1110 radio chip, a favorite of experimenters and hobbyists, normally costing at least 3 or 4 times the total purchase price of the entire unit! For those so inclined, try Googling "Hacking the Girl Tech IM-ME" for many blogs and forums with plans and info.