I am surprised that Mcafee charged you whilst asking you to install a third-party tool which you can download for free, not sure how Malwarebytes would feel about that?
Without knowing what you were infected with, it is hard to give an answer as to why it was not detected by Mcafee, but, the rule is that no vendor will give you 100% coverage- so doing online scans every month with another vendor like Eset, Kaspersky or Panda to name three is always a good idea. Furthermore, since no vendor will offer 100% coverage, installing programs such as Malwarebytes and Supernatispyware is also a good way of adding an extra layer of protection
There are plenty of good trustworthy sites that offer free virus/spyware/malware removal, such as the ones below:
Thank you for listing the other sites. Sorry if I was not clear and, in addition, misspelled one of the antivirus software products mentioned. Each of these services used various antivirus products to clean my computer. In each case they took control of the computer and installed and used Malwarebytes as well as other software to clean my computer. Again, it seems to me that when I ran the updated full scan McAfee before I phoned these services, McAfee should have reported some of the stuff that these other software products found. You say that no vendor will offer 100% protection and I understand, but in these two separate cases when the software they used detected so many items and threats detected, one would wonder about the efficacy of McAfee.
Without knowing what the other software found, it is hard to know whether what they found was malicious or not, some vendors will say a file is malicious others will say no malicious code found.