Typically this is due to having made the original purchase under an older / other google account.  

One solution is to setup an additional google account for the account used to originally purchase the app and use that to get the update through the market.   This is simply a matter of accessing the phone's setting, choosing "accounts" and setting up the previous account the apps where purchased under.   Then from within the Market switch to that Google account.

http://www.google.com/support/androidmarket/bin/answer.py?hl=en&answer=113410

The second solution is to verify the purchase with us by sending either the order number or full name the app was purchase under and we will setup a download from babyesp.com.  Just drop us a note at support@babyesp.com for assistance with this option.

Best Regards,
Matthew S
BabyESP Support