Once you created a material (or selected one from the pre-existing ones), you need to scroll down past the item's properties, and you'll arrive at the dilutions section. There you'll find 2 fields:

• %: the percentage of material used in the dilution
• Add diluent: select (or create) a solvent with which you dilute the material

Once the above two fields are filled, press the tick, and the dilution is created. The dilution will inherit all properties of the item that has been diluted.