Not sure if LR2 can handle ACR 5.4. Website states it is only for CS4.
Anyways, fyi, I recently got the EOS 500d and as my CS3 does not support ACR 5.4, my work around was to download the latest DNG converter,
convert my CR2 RAW files to DNG (to be readable for ACR 4.6) and work on the resulting DNG file in CS3.