For the cpu temp lead/probe, you want it as close to the processor as you can possible get without actually preventing the heatsink and fan from doing their jobs. AS for the hard drive probe, let the drive run for a while and feel around for it's warmest spot... Place the probe there so that you'll get the highest temp reading on the drive.
Personally, I've found those types of probes just shy of totally useless. For one thing, only extreme temps (for the innards of a computer case) will do harm to a hard drive. For another, the drives are made to shutdown if they hit the danger zone as far as temps go. As are most computer components/parts. Motherboards have probes that will give you a reading built into them. You can use software like Motherboard Monitor to display the temps in your system tray, so you don't need to look far for them.