Depending on your OpenERP, but assuming v7.

Go to the access rights and make sure that the user has access to different UoM's. Also check sales/purchase under settings to see those optiuons are checked (I believe they are called somerthing like "allow multiple uoms on product").

If that all is checked and done, go over to your product. You should now see a distinction between Sale and Purchase UoM. Supply the required UoM's there.

If the UoM does not yet exist, make sure that under sales --> configuration --> products --> product Units of Measurement the UoM you require exist. 

The right UoM should be selected when you pick this product on either a sales or purchase order.