ModelSim Altera: simulating the "lpm_add_sub" module?
According to Altera's knowledge base, it's a bug in their simulation library, which incorrectly has the parameter names defined only in lower case (it should, apparently, be able to accept either).

http://www.altera.co.uk/support/kdb/solutions/rd04192000_5867.html

Searching directly for the error message didn't turn it up, because the text of the error message appears to have changed since they added the description of the problem to their knowledge base.

Fortunately, for synthesis the lower case names are accepted just as the upper case ones (as used in the documentation) are.