The interface to your right will allow you to upload a mission to The Citadel ArmA 3 server. The mission must be in .pbo format in order to function once on the server.
Click the "Choose File" button. This will open an Explorer window and allow you to navigate to the location of the mission. Select the mission and click Open. With the mission selected now click the "Submit" button. A gear animation will appear on screen and the progress bar under the submit button will show upload status. Once the upload is complete at 100%, the page will refresh back to its original state.
The mission is now uploaded and playable on The Citadel.