lpm_add_sub synthesis error on Q2 9.0 SP2

On Flex 10k devices I sometimes had problems where Quartus would refuse to compile for something that was "illegal in the device" but Max Plus2 didnt have a problem. Have you tried older versions of quartus or max plus 2?

As Acex is such an old part, and there are very few people that actually use AHDL any more (altera recommend you dont use it) you may struggle to get much help on the forum. Have you tried contacting altera mysupport?