How to add subtypes to existing classes

[ShaningYu]

I updated an existing GDB with creating a dozen of coded domains.  Then, I planned to add Subtypes in two ways but not successful.  The 2 approaches I used are described below:
1)  Right-clicked a existing feature class for its properties.  However, the Subtype Field's combobox is disabled. 
2)  Tried to use the Subtypes: Add Subtype tool.  But the feature class loaded in the Input Table displays the Red-Cross sign (Not available).
Please share your experience if you can.  Thanks.

[JakeSkinner]

Does your feature class contain a field of type 'Long Integer' or 'Short Integer'?  This is required when creating a subtype.

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//005r00000003000000.htm

[DarrinBlaisdell · Accepted solution]

If you have grayed out buttons (the Apply button) there is probably a shared lock on the feature class you are trying to add subtypes to.  You need an exclusive lock, meaning you are the only user accessing the feature class, before you can add/modify subtypes and apply those changes in the geodatabase.

-D