You cannot designate that this blocking factor has a random effect in the design. But you can when you analyze the data. Select Analyze > Fit Model. (It should be populated if there is a Model table script, otherwise, assign roles and add effects as necessary.) Select the lone term for the block effect, click the red triangle next to Attributes, and select Random Effect.
To avoid having to perform this step each time you open Fit Model dialog, click the red triangle at the top and select Save to Data Table.
