Level Your Frame Level.
Is your frame on a sloped patio? Maybe it is on Pavers?
We have had hundreds of clients that have leveled their frames on these type of surfaces. They use a combination of composite shims, cement board strips and decking board strips.
You can use exterior construction adhesive to glue decking board strips together. This is great for the end that needs 1-3 inches.
For under 1" use 1/2" cement board or 1/4" shims.
Once you have the frame level you need to secure the frame to the concrete with either the blue masonry screws or Red Bolts. Both are sold at Home Depot or Lowes.
The reason you secure frame to the concrete is so that it never slips off the shims.
If your frame is level these islands sit on their own weight and do not need to be bolted down. Some of our Customers in Florida bolt them down due to hurricanes.
Some people buy leveling feet that are rated for 800 plus pounds.