Once again, "we" cannot "fix it" for you. This is a customer-to-customer forum and, while DreamHost staff checks in on occasion, they rarely post here and this is not a way to reliably reach DreamHost Support.
The PHP bcmath "problem" is not one that DreamHost support can/will help you with. They do not offer technical support for installing third-party scripts. That is why there is no "category" for your problem.
As Mousee has told you, you can contact the technical support staff via the Control Panel -> Support -> Contact Support page but, as you correctly point out, there is no category for your problem.
The default Dreamhost PHP installations do not enable the bcmath module. In spite of the directions/error message generated by the script, DH does not install custom PHP configurations for customers' use.
The message "Please contact web server administrator/web hosting company to enable the module," is a good enough suggestion, but will not help you in this instance, as I have explained above. If you want to enable the bcmath module, you will need to do it yourself, or find someone to do it for you.
DreamHost allows you to install your own version of PHP, and you may certianly enable the bcmath module in such an installation if you choose to do so.
You can learn more about enabling the bcmath module at:
Here are several DH wiki pages that can help you understand how to do this in the DreamHost environment:
If this is not something you feel comfortable attempting yourself, you might find someone else to do it for you; I know Mousee has indicated he does such things.