How to add an extended property to a table type?
The MSDN documentation:

http://msdn.microsoft.com/en-us/library/ms180047.aspx

appears to be incorrect.  TABLE_TYPE is not a valid level1type value.

Documentation is also missing here:

http://technet.microsoft.com/en-us/library/ms190243(v=sql.105).aspx

but a comment at the bottom provided the answer.

Extended properties may be specified for a table type by using a leve1type of 'TYPE'

EXEC sys.sp_addextendedproperty 
@name = N'MS_Description', 
@value = N'An INT value.', 
@level0type = N'SCHEMA', @level0name = dbo, 
@level1type = N'TYPE',  @level1name = AllTypesRecord,
@level2type = N'COLUMN', @level2name = 'Id'
Go