I was playing in the arena, and I tried to bring in an Iron Dwarf (battlecry +2 to Attack) I had nothing else on the board as my oponent had just wiped me, and that was the only card in my hand. I could not target the Iron Dwarf himself, but I could not find any way to just let the battle cry lapse and let him come into play. I probably would have lost either way, but I was wondering if there was a way I could have brought him out onto the board. The only other possibility seemed to be to apply the +2 to the opponent's minion, but that seemed even less optimal.
As has been alluded to, if it is possible for you to use your battle cry, you must. In this case, your thought was correct - you would have been obligated to use it on an opposing minion. However, the iron dwarf battle cry is applied only to this turn, meaning your turn. You would have used the battle cry on an opposing minion, only for the effect to wear off when the opponent's turn began. In this case, you wouldn't be "punished" for using it.
If you're playing on PC, you just drag the card onto the battlefield and it won't allow you to target anything. If there IS a target available, you'll have to target that minion, even if it is the opponent's minion.
I'm not sure how it works on tablets, but I'm sure it's the same thing.
There is nothing special you have to do to make it summon, just drag it onto the field.