Cubase 4 MPEX timestretch option missing

Ive been using cubase 4 for ages now and the mpex algorithm for timestretch. After a little break Ive gone back to use the program and found that in the timestretch dialogue box the MPEX option is missing. I can only use the realtime algorithm which is really frustrating!!!

Im starting to pull my hair out with this now, has anyone experienced this and can help???