The instructions are in the readme.txt file that's included in the vBMail BF download. If you've opened that and have questions still, can you please be more specific?
I am unable to replicate any issues with purchasing both a 3 month vBMail and a vBMail Branding Free licence simultaneously.
Do you remember the exact steps you took, as well as the error message you received? Having this info will help me immensely in attempting to track down this problem