If it is already connected, unplug the printer's USB cable from the Time Capsule as well as your Mac since it will just confuse matters.
To add the printer to your wireless network, the printer must first "join" that network. That is sometimes accomplished with a printer's front panel setting but I do not see one in the C4780's documentation.
When you print to that printer wirelessly, you will need to select a different printer in any OS X Print dialog box. It may not exist yet, which means you have to add the printer in System Preferences > Print & Scan. Read this to determine how to do that:
Mac OS X v10.6: How to connect a wireless-capable printer to your AirPort network
If all goes well the Mac will "see" the printer as an available wireless printer, automatically select the appropriate driver, and add the printer to Print & Scan.
When attempting to print to this newly created wireless printer queue, and you select the printer that you use when the printer is connected with a USB cable to your Mac, it won't find the printer. This is probably what you have been experiencing. A different connection method, e.g. wired vs. wireless, is a completely different printer as far as the Mac is concerned.
For an overview of printing in general, including information regarding configuring a new printer:
Mac 101: Printing (OS X Lion)