It depends on the iPad's connection port. I have an older iPad with a 30-pin port, so I use a 30-pin Camera Connection Kit. If you have a newer iPad with a Lightning port then you'll need a Lightning Camera Connection Kit. Depending on where you live, there may be a store in your area that carries one-- e.g., in America they're usually readily available at Walmart, Target, Best Buy, etc.
You should have gotten a USB cable with the Alesis, but depending on where its USB-MIDI port is and where you're going to put the iPad, you might want to buy a USB cable of a different length-- e.g., if I'm using my iPad with one of my Yamaha keyboards, I like to put the iPad on the keyboard's music rest, so I use a short USB cable because that keeps a lot of excess cable from hanging down behind the keyboard.