Lighting a portal in Minecraft

As of Version 0.12.0 Alpha release on July 29th, 2015 Tyler Dood's Answer is correct.

Before this however the reason why you can't get the portal to work is because in Pocket Edition The Nether does not exist as such you can not create a portal to it.

There is no Nether in Pocket Edition. There is, however, a structure known as the Nether Reactor. It is built from 14 blocks of cobblestone, 4 blocks of gold and 1 nether reactor core.

As per the quote above to access the equivalent in Pocket Edition you need a Nether Rector,

The nether reactor is a built structure which, when activated, generates a massive structure of netherrack (35×17×17) with multiple floors called a "nether spire" (unofficial name). It also turns the reactor itself, excluding the core, into glowing obsidian. At this point the reactor will begin to spawn various blocks and items, as well as zombie pigmen (if difficulty is up)

As for the Nether Rector post 0.12.0 Alpha, under Changes -> Block and Items

Still usable, but is uncraftable and removed from the crafting menu. If the player breaks a Nether reactor core, it still drops 3 diamonds and 6 iron ingots.

While all other items normally generated by the reactor can be found normally in The Nether

Make sure the portal looks like this:

The dimensions are 4x5, or you can even exclude the corners. If it still doesn't work, try breaking one block and placing it again. The frame can be built up to 23x23 blocks, but must be no smaller, and in the correct "upright" frame (4x5).

Hope this helps!

The portal is 4×5 obsidian. When ignited by flint and steel, the purple portal apears. If you have minecraft pocket edition 0.12.0, then for the portal to work it must be an infinite world. When you make your new world you can press a button in the corner. It says "advanced". This is where you can chose if you want your world "flat" "infinite" or "old". Chose infinite and your portal should work if its built correctly.?

You have to go back to title click on play look at the world you want the portal on. Click on the small pencil next to it click on make infinite, and finally click on make always day. This should work and I hope it helps!