-You can´t have Cubase on an iPad
-Install and activate one copy on one computer
-install the other copy on the other computer
-or install one copy on both computers, and move the license to a USB elicenser.
That is Cubasis on the iPad, not Cubase, they work together but are different animals, I do not think that there is a free version of Cubasis included with either of the products you bought. The reference on the Yamaha site is to the hardware interface itself, not Cubase, i.e. the interface works with the iPad, there is no mention of a free Cubasis or of Cubase existing for the iPad.
There should be a activation code for Cubase with the zoom h4n you bought, if it is not on the CD or in the box and you bought it new, you should ask the company that sold it to you to provide you with one, if you bought it second hand you will need to ask the person that sold it to you to de-register the Cubase LE/AI copy so you can register it on your computer.