Sorry for the delay setting up this board - I missed your original post for some reason.
Anyway, those messages you are seeing are a result of running nBill on PHP5. Whilst nBill will work with PHP5 (as long as you suppress warnings), it was written for PHP4. Some notices and warnings that PHP5 generates are unavoidable without a complete re-write, so they just have to be suppressed/ignored I'm afraid.
It is possible that the next major version of nBill (2.0) will be written specifically for PHP5.