What type of AMEX do you have?

• If you have the Starwood Preferred Guest AMEX you can transfer starpoints to United at the rate of 1 mile per 2 starpoints.
• If you have Blue, The Knot, the Nest, or similar AMEX credit cards you are in Membership Rewards Express and can't transfer points to any mileage program.
• If you have Green or Gold Amex charge cards the you are in Membership Rewards. You can trasfer points to Delta, Continental, stuff like that, but not United.. Same with the Platinum charge card in MR First.

You can get United flights on the pay with Points option, but can't transfer MR points directly into Mileage Plus.