use the sp_addextendedproperty SP to add description to a column.

sp_addextendedproperty
    [ @name = ] { 'property_name' }
    [ , [ @value = ] { 'value' }
        [ , [ @level0type = ] { 'level0_object_type' }
         , [ @level0name = ] { 'level0_object_name' }
            [ , [ @level1type = ] { 'level1_object_type' }
             , [ @level1name = ] { 'level1_object_name' }
                    [ , [ @level2type = ] { 'level2_object_type' }
                     , [ @level2name = ] { 'level2_object_name' }
                    ]
            ]
        ]
    ]

e.g.

EXEC  sp_addextendedproperty 'MS_Description', '<column description>', 'user', dbo, 'table', '<table name enclosed with single quote>', 'column', <column name not enclosed with single quote>

you can use anyname on [value] but if you want to see the description on Enterprise Manager, use 'MS_Description'.

to view the column properties, use the fn_listextendedproperty function.

e.g.

SELECT   *
FROM   ::fn_listextendedproperty (NULL, 'user', 'dbo', 'table', '<table name enclosed with single quote>', 'column', default)

BOL: Property Management