This is from the Designer Help:
1. A document must be open in Read or Edit mode.
2. A form or subform must be open in Design mode.
3. An attachment must be selected.
This @command does not work when used in a hotspot button. The hotspot button takes focus away from the attachment that must be selected to be viewed. Use in an action button instead.
This command does not work on the Web.
As you can see you have to select the attachment before this command can be run. You have to select the attachment before you run the @Command. That is most likely what your problem is.
Good luck in solving this one. Depending on exactly what you are trying to do you may need to write the code as LotusSctipt.