Step 1: Open Minecraft Step 2: Click "Multiplayer" Step 3: Click "Add server" Step 4: In the second box type "skyblock.net" Step 5: Click "Add server" Step 6: Join
If you mean you have a modded client, do not worry, you should still be able to join the server using that client as long as it's in 1.8 or above. Make sure it doesn't have any illegal hacks on it; heres a list: Acceptable Hacks/Cheats | Skyblock Forums