Did it ever work? The microSD holder on the DSTT is spring-loaded (I think that's true of all versions, including the fakes). Sometimes people report that they get one that isn't spring-loaded, and it turns out the spring was just "stuck" but usually comes unstuck after inserting the microSD card a few times.
If you have a newer one with a divot where the microSD slot is, you can usually pry it open (it snaps shut). Older models are glued shut and require more care (and sometimes an X-acto knife) to open, and some glue to close again.