"It asks me to restart from a DVD or something, how do I do it? I want to defrag the whole drive, it is really slow."
You can't do a "full defrag" on a volume that you're booted up from. It's like asking a brain surgeon to do surgery on himself.
You have to boot from _another_ volume that has iDefrag on it, and then "aim" iDefrag at your "target volume", and let iDefrag do its thing.
If you don't already have a _bootable_ backup volume, I'd STRONGLY suggest that you consider creating one. It's not hard, nor is it expensive.
What you might consider is one of these gadgets:
http://www.amazon.com/s/ref=nb_sb_noss?field-keywords=usb+sata+dock&url=search-alias=aps&x=0&y=0
(many items shown, they all work the same, just pick one you like that is cheap)
.... and then, get a "bare hard drive" from the vendor of your choice (I like newegg.com, or you can check dealmac.com for deals).
Put the drive into the dock, connect it to the Mac, boot up, initialize the docked drive.
Then, use the FREE "CarbonCopyCloner" from:
http://bombich.com/
... to create a "clone" of your current internal hard drive (which probably already has iDefrag on it).
Now, re-boot from the docked drive:
- Press the restart button
- As soon as you hear the startup sound, hold down the Option key and KEEP HOLDING IT DOWN
- The startup manager will appear
- Select the docked drive, hit the enter key, and the Mac will boot from the dock.
When you get to the finder, you are now booted from the docked drive.
STRONG SUGGESTION: Change the desktop picture of the docked drive so that you can easily distinguish it from your _main_ drive (and keep from getting confused).
Launch iDefrag, aim it at your main drive, and go.
Works great for me.
One last thing:
IGNORE the comments saying that you "don't need a defragger with OS X". Your Mac _will_ run better after a defrag!