How to make a lever to open iron doors in minecraft PE?

They haven't added redstone yet.

You can't in pocket edition- while the 'Iron Door' item itself exists, the mechanisms for opening it have not yet been added, as with everything else redstone related.
Redstone and redstone-based items exist, but can't be placed yet, which is what I mean by redstone supporting mechanisms.

The Iron Door itself shouldn't normally be available to players for this reason. People would complain if the item was added to the normal game and didn't have any of the supporting items to make it of any use whatsoever.

You're either encountered a glitch by making the item, or have purposefully edited your inventory to get ahold of the item. Either way as it is now it's useless until new updates come out (unless you want to use it as decoration).

The next update is 0.12.0, but unfortunately this doesn't appear to be adding the ability to use redstone. Who knows though, you might be lucky.

^-^ Hope this helps! ^-^

Put a piece of cobblestone on the bottom and one stick above it.

Minecraft pe doesn't have red stone yet