As jessedlc21 had mentioned, you only receive a Masterball when using the Complex Pixelmon Modpack. The Masterball is a one-time gift that only appears on the first server you join with the Modpack. It typically takes a few minutes for the system to process it and will automatically give you the Masterball, given that your inventory has an available slot. However, if it takes more than 15 minutes, I will gladly check the status of your Modpack reward. Please list your IGN (in-game name) below and I will get back to you as soon as possible. If there is anything else, please feel free to let us know and a staff member will help you out as soon as possible! Stay safe and healthy.