Can you take a pic of how it's set up? Especially if you can show the highest water level in the sump in relation to the neck of the skimmer.
You should be able to adjust it with the valve on the output of the skimmer. If it's open all the way and it's overflowing, the feed pump is probably too strong. What are you using to feed the skimmer? You can always just go to Lowes or Ace and get an inline valve for after the feed pump to slow down the water going into the skimmer.