I am using Ubuntu 12.04 (64 bits) and LibreOffice Version 4.0.3.3 (Build ID: 400m0(Build:3)).
The problem that I have come across is quite funny. when I use Insert>Object>formula for first time it works, then when I want to use it again in same document continuously, it appears disabled. But you close the document and reopen again, it gets enabled. Though this small trick serves as of now, but closing and opening documents to enter many mathematical formula is not really conducive.
I just hope someone will fix this small problem, so that OL improves for betterment.