Each time you start iBank, it should automatically display the last data file you opened. iBank relies on Mac OS X "Recent Items" to remember the location of your data file - if you have this feature disabled, iBank will not be able to open the file automatically. To make sure you have not disabled this feature, open System Preferences and click the "Appearance" button. Under "Number of Recent Items," make sure you have a number selected next to "Documents" and not "None."
Some third-party menu utilities may interfere with the "Recent Items" feature. If iBank is not opening your data file automatically and you have one of these utilities installed, you may need to reconfigure or uninstall the utility.