some info I have been prompted to pass on,,,,
First: None of the bridges will work with SMF 2.0 RC3. Until there is a release with the integration hooks fixed, they just won't work. So upgrading to RC3 is a bad idea, regardless of security. Just having the forum integrated in Mambo and not accessible directly will help with most XSS vulnerabilities, anyway. All input is passed through Mambo and cleaned before SMF gets a chance at it, so there's a better chance at averting XSS attempts.
Second: It seems that your problem is likely to be database permissions, not files (as I first suggested). The error is a Mambo-generated error, not a PHP error. Keep in mind that on a local WAMP server, if the database user names are different, it won't actually perform the switch properly. (and sometimes, even if the database users LOOK the same, they are actually different on many of the non cpanel site panels)
Third: Make sure you have the SMF_header_include mambot published. This is very important.
Fourth: You are using bridge registration. That component was never updated for the 2.0 bridge, and should be deprecated by now. Most users choose SMF or Mambo registration, anyway. Mambo registration with the SMF_register mambot published is the most secure method.