Can you locate the file om_assemble and issue the command:
ldd om_assemble
There should be a line saying:
libOpenMEEG.so.1 => Not found
What is the content of the line libOpenMEEGMaths.so.1 => ?
Normally, these libraries are provided with the package and you should find them not very far from the place where you found om_assemble (presumably in the same folder if I remember well). Do you have them ? If yes check their permissions...
If you do not know where to look for om_assemble try:
locate om_assemble (I do not know by heart where BrainStorm places OpenMEEG).