Trying to fill in the blanks a bit: are you asking how to create a world using a seed, like "OMGThisSeedIsAwesome", so that you can play on the generated world?
To do that:
1) Select New Game
2) In "Game World" select "NEW RANDOM WORLD"
3) In "World Generation Seed" type in your seed value
4) Select the world size you want. NOTE - if you are trying to recreate a world that someone else gave you, you MUST use the same world size!
When you click "Start" the game will generate the new world for you. Might take quite a few minutes depending on the size.
(I'm running SMX UI so the screen below will look different from yours, but the options are the same.)