Bulk edit tables and columns descriptions
You can't add them in mass but you can generate the commands to do so pretty easily.  I've added two extra columns to the query that have the sys.sp_addextendedproperty commands added if there isn't already a description.  You would still have to manually modify the description value for each but this should save you some time.

SELECT      u.name + '.' + t.name AS [table],
            td.value AS [table_desc],
            CASE WHEN td.value IS NULL THEN 
                    'EXEC sys.sp_addextendedproperty @name=N''TableDescription'', ' + 
                        '@value=N''This is the table description'' , ' + 
                        '@level0type=N''SCHEMA'',@level0name=N'''+QUOTENAME(u.name)+''', ' + 
                        '@level1type=N''TABLE'',@level1name=N'''+QUOTENAME(t.name)+'''' END AS CreateTableNotes,
            c.name AS [column],
            cd.value AS [column_desc],
            CASE WHEN cd.value IS NULL THEN 
                    'EXEC sys.sp_addextendedproperty @name=N''ColumnDescription'', ' +
                        '@value=N''This is the column description'' , ' +
                        '@level0type=N''SCHEMA'',@level0name=N'''+QUOTENAME(u.name)+''', ' + 
                        '@level1type=N''TABLE'',@level1name=N'''+QUOTENAME(t.name)+''', ' +
                        '@level2type=N''COLUMN'',@level2name=N'''+QUOTENAME(c.name)+'''' END AS CreateColumnNotes
FROM        sysobjects t
INNER JOIN  sysusers u
    ON      u.uid = t.uid
LEFT OUTER JOIN sys.extended_properties td
    ON      td.major_id = t.id
    AND     td.minor_id = 0
    AND     td.name = 'MS_Description'
INNER JOIN  syscolumns c
    ON      c.id = t.id
LEFT OUTER JOIN sys.extended_properties cd
    ON      cd.major_id = c.id
    AND     cd.minor_id = c.colid
    AND     cd.name = 'MS_Description'
WHERE t.type = 'u'
ORDER BY    t.name, c.colorder