If you find that your testicles are too close to the base of the Penomet cylinder or that they get sucked into the cylinder during pumping, this can potentially cause discomfort and loss of suction during use of the device.
To overcome this issue you should first ensure that you use warm to hot water and allow your body to adjust to the temperature of water prior to beginning a pumping set. This will allow the scrotum to loosen, decreasing the risk of the scrotum being sucked into the cylinder during a session.
Although the above will help, you may potentially find that your testicules still become sore and/or get sucked into the cylinder. As an additional solution you can use a ball stretcher or cock ring to isolate your testicule sack further from your body.
For additional comfort and suction, some users also find the use of a universal pump sleeve of benefit which can be purchased from the likes of Ebay and a variety of popular online adult product stores.