To be specific Open PSE-->select Unit of Measure (bottom left)--> Click on Class --> right click New Unit of Measure Class if you want to create custom class e.g. Pressure, Temperature(which already exists).Once Custom UOM Class is created then right click New Unit of Measure --> General Tab --> Name , Abbreviation, Reference UOM (if any) . Please note you can also include new UOM in existing class e.g. Pressure where you can define your UOM and select relevant reference.
If you want to include some formula then click on formula and add input and output and select UOM Group mapping if any.
Once checkin in PSE you can use in AF attributes.
Additional Reference : PI Server