You will need to create a new world, as each FTB pack uses a separate save folder. In addition, many major menus have been overhauled this is a feature of VoxelMenu, and should not be cause for alarm. Once you have successfully installed the FTB launcher, sated your curiosity on the wiki, and selected the Yogpack from the list of available modpacks, choose "launch." You should notice a custom Feed the Beast intro animation. In addition, you may wish to peruse the YogCraft Pack page for additional info about the pack itself and its attending mods.
If you are having difficulties, please see Tutorial:Installing Feed The Beast on PC and/or Tutorial:Adding Optifine.
This guide will assume that you have successfully installed FTB (and Optifine, if you so choose).