The Mac will not "find' the printer unless you.....
Open System Preferences (gear icon on the dock)
Open Print & Scan
Click the + (plus) button at the bottom of the printer list on the left
Click Add Printer
Look for you printer on the displayed list
Click to highlight the printer
If you have previously installed the printer when it was connected to the USB port on the Mac, then it will appear as if your printer is installed two times in Print & Scan.
You must choose the correct printer....the one intalled at the AirPort Express.....when you try to print
Not all printers are supported. If your printer does not appear when you click Add Printer in the steps above, then your printer may not be supported.
If you can get the printer working with the Mac, then you can begin to work on enabling the iPad to print. To do this,you will need to install an App like Printopia. There is a free trial, so you will know in a few minutes whether you will be able to print from the iPad. More info here: