I have a 690 case, so i will try to explain it the best i can from memory...
If the side of the case is off and it is laying down on the ground in front of you, to your right, near the front of the case on the bottom-ish section are small black trays.
To pull the trays out, just sqweez the semi-circular handle and it should slide right out. You should notice on the black tray, around the inside, there are 4 little prongs, those 4 prongs go where the screws would normally go on the HDD. So go ahead and place the HDD in the tray, face up, making sure the prongs line up in the hole.
You should notice the prongs are rubber coated at the base, this is nice because it cuts down on vibration.
Once the drive is in the tray, slide the tray back into position and it should be fine. Connect the SATA or IDE in the back and the power and you're good to go.
If you only have 1 HDD, i would recommend placing it in the center of the rack that way it gets the most air coverage (it doens't ~really~ matter, but it wouldn't hurt) you may also chose to pull out all of the unused HDD trays that you aren't using so they don't block the front intake air flow at all.
Here are some pics to hopefully help, this isn't my CM690, but it's a good clear shot
Optical drives are always 5¼". The two size names go back to the early days of floppy disks. Originally IBM PC's (and many home computer systems of that time) came with floppy drives that used disks 5¼" diameter. The computer case had one or more mounting bays open to the front in which to mount the floppy drives. When IBM introduces the PS/2 design in the mid-80's they also brought in a new floppy disk system using disks of only 3½" diameter inside a hard plastic case with a sliding cover over the access window. They had higher storage capacity, were better-protected, and took up less space. The mounting bays for them, of course, were smaller, too. That was also the time that hard drives, too, adopted a physical case size similar to the 3½" floppy drive. However, certain devices like backup tape drives and, later, optical (CD-ROM) drives needed the larger space of the older 5¼" drive case and bay, so both systems have been maintained.
In current systems you will find hard drive internal (unexposed) bays of 3½" width only, but front-accessible bays of both 3½" and 5¼" size with removable covers. In them you can mount 3½" floppy drives (nobody uses 5¼" floppies any more) and a few similar devices and optical drives in the 5¼" bays. Those wider bays also are used sometimes for accessories like multi-card memory chip readers or fan controllers.
Old guy time: anyone here remember using 8" floppy disks?