The replacement battery sold by REI is for the older Fastfind models, not the 210. The REI website specifically states that the battery pack "is not compatible with the FastFind 210". Look at the last bullet point and NOTE.
http://www.rei.com/product/791973/mcmurdo-plb-replacement-battery-pack
The battery for the Fastfind 210 is non-user-replaceable, so you'll have to mail or bring it to one of McMurdo's authorized service centers in the US. They work with Survitec Survival Products Inc., which has a bunch of local dealers. Not sure on the cost though.
http://www.mcmurdo.co.uk/where_to_buy/product_service.html?product_type=2#
http://www.survitec-spi.com/service-stations
