I hit the same problem using Lbook software - see
https://www.mobileread.com/forums/showthread.php?t=46382 .
The issue seems to be the difference between .idx and .index files that are used with the .dict.dz file.
As I mentioned in the other thread the mueller dictionary that came with the Lbook s/w does work (but translates English to Russian) so pehaps it is just a matter of generating the correct .index file .
BobC