I bought business software and the software company is dictating to me who I have to use as my credit card processor, is this legal?

What are the terms of use for the software? (Be sure to check the fine print, too.) If the terms allow them to so specify, they can do so--there is nothing illegal or unenforceable about selling a product subject to it only being used with certain service providers or vendors.

Of if the product requires any ongoing interaction with the software company--e.g it works on their website, through their service, with a connection to their computers, etc.--they probably can dictate whose other systems they will or will not support or work with. Possibly if there was nothing in either the terms or marketing materials indicating that this was a possibility, you may be able to state a case for a refund; I doubt very much you can force a business to work with partners it does not want to.