No land to start on Agrarian Skies?

After looking around a bit, there appear to be two possible reasons for that.

Firstly, selecting a new world does not work, since the skyblock is not generated like that. If you don't have a savegame already for some reason, you can download the map here. Put it in your saves directory inside your directory for AS, launch the game and select that world.

If you use that savegame and still spawn in the void, here's a quote by jadedcat herself:

If you spawn and fall through the void swap to creative and fly up. Use /jspawn set to set the spawn to the platform.

The easier way, on the Maps & Textures tab of the FTB Launcher are the maps available to download. By selecting your Agrarian Skies mod pack in the bottom pulldown (next to the Install button), it will install the maps for you. FYI, you may have to click the Maps button on the top as it seems to default the Texture Packs list.