You have not provided nearly enough information for anyone to try and diagnose and/or troubleshoot your problem.
What is the exact brand and model of the eSATA expresscard? What is the exact brand and model of the hard drive you're trying to connect to it? What steps have you already taken in trying to get them to work together? Can Disk Utility "see" the drive? Do any error messages pop up when you connect the drive? Is the drive even plugged into a power source? Are you sure the hard drive is eSATA, and not USB 2.0 or FireWire?
If you could elaborate (greatly) on your situation, we can better try to help you.