@gerrr00:
To use an Omni plant, just click on it and select the "Feed" option. It will pull up a list of every item in your inventory that can be fed to the plant. One word of caution, though... if you have a ton of things in your inventory, it will be slow to load because it loades every item individually. For example, if you have 50 tomatoes, there will be 50 lines of just tomatoes.