Wide MACCs in Stratix III

I'm trying to build a 24-bit accumulator in Stratix III. I don't actually need a multiply function, but I'm using the DSP block anyway to be assured of fast operation. So, I'm taking my 24-bit value, and multiplying it by either 1 or -1 to add or subtract from the running total.

The problem is, I cannot get any DSP Block-based accumulator wider than 18 bits to synthesize. I always get the following error message:

Error: Can't elaborate user hierarchy "altmult_add:altmult_add_component"

I've tried a number of different approaches, from the straightforward attack of asking the Megawizard generator to build a 24-bit MACC, to various sneaky approaches where I break the 24-bit number into two chunks, and feed it into two separate multipliers, each feeding the same accumulator. No dice.

Any ideas?

Thanks,

SL Boarder