optiplex 7040-tower 3.5" install question-- What's the secret?

There is no secret.  After pulling the HDD caddy tray out toward the front of the PC, just align the mounting holes of your HDD to match with those pin grommets.  Take note that each grommets pin have the blue tab on the caddy, press and pull them outward until the metal pin could insert into the HDD mounting space. 

if you have the 0CW33 hdd caddy for 7040 MT

the grommet can shift in the oval hole and go out of alignment with 3.5 hdd side holes.  you can look at pic of caddy and make sure all 4 grommets align w hdd side holes

redxps630: tyvm for the photo

this HD slot was empty, and i initially  thought (as used to be) they were screws, so I took them out.

and then installed onto the hd, and tried the wrong way to insert them back into slot.

your photo and suggestion would be quite good for the online documents (if only they would update this old thing-)

Will try again when I remove case again.

Thanks

yeah the grommets design is not optimal when they can shift or bend out of alignment.  I find them sometimes user unfriendly.