This is apparently a really well known issue with SQL Server Management Studio. When you try to add “newsequentialid()” to the “Default Value or Binding” field in the designer it keeps throwing the error “Error validating the default for column ”. Do you want to cancel your changes?”. The method I found to work around this is shown below using T-SQL instead of the designer UI.
ALTER TABLE dbo.TableName ADD CONSTRAINT DF_ColumnName DEFAULT newsequentialid() FOR ColumnName