Hi so I threw some of the seeds on the ground and they disappear after a short amount of time but they don't actually plant.... Am I missing something?

That's not how spreading works... They can naturally drop from a tree and plant themselves into the ground - not from a player dropping them on the ground. If you want to plant a new tree yourself, you can right click the ground with a seed like you would with a vanilla sapling.

To amend my previous statement: seeds do have a chance to plant themselves if you drop them on the ground AND they land in a forest biome (if the seedOnlyForest config option is disabled you can override the latter).