You can't install ADE on ipad.
If you want an ereader for your ipad, you can download any of the iOS application mentioned on the below link:
Search for iOS Application under the column Application or Mobile/tethered device. For example you can download DL Reader for iOS.
All these applications support the .acsm ,and also, you can transfer the fulfilled books from your PC to your device and open them in these apps.(using the same Activation ID)