If your worlds are nothing but voids when created then it's the Ex Nihilo mod which is causing it. The mod is vital for Agarian Skies survival and progression. To enable overworld generation, just go into "crowley.skyblock.cfg " and change " B: Overworld=true " to read " B: Overworld=false " under " # void generation options " and create a new world. Doing so will ruin your Agarian Skies experience as there will be a "mainland" when you play the map.
As for additional mods, the only mod I added to the pack is " InGameInfo by DaftPVF. " It just gives me what info what i need at a glance.