They are pre-charged, but you will need to use a vacuum pump to vacuum the lines of air once everything is connected up. This also allows you to check for leaks, as the vacuum should hold for over an hour once it's pulled down (If you see gauge moving back up, it's time to go back and check your connections) Once the lines are vacuumed down and all air/moisture is removed, you can then open the valves on the outdoor unit and let the refrigerant into the lines and the indoor unit...the system will then be ready for use.
If you want to install the system yourself, you will need to borrow or rent a vacuum pump. You should not need to add any charge to the system. Most mini-splits have enough charge for a 25 foot lineset. A longer lineset will require more refrigerant than what's contained in the outdoor unit's compressor.
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