Tell me what version of OS etc. you are using, give me detailed step-by step instructions about what you are doing and I will see if I can help you. Be sure to install phpBB first and then Zen cart, be sure to also use the same mysql database - using different prefixes (phpbb_ and zen_).
Let me know how you get on. I am going to New Zealand in 10 days so will not be available for a week after then (just in case my correspondence goes silent).