To make backups of your MacBook Pro, you can use Time Machine > http://support.apple.com/kb/HT1427 To set up Time Machine, open System Preferences > Time Machine > Select Disk, select the external drive and the first backup will start automatically. Time Machine will make hourly backups.
If you have any question about Time Machine, visit > http://pondini.org
There are other ways to make backups of your MacBook Pro, but the easiest and one of the most reliable ways is to use Time Machine
If your external drive is connected through USB 2.0, the backup may take hours because it's the slowest port. If you are using FireWire or USB 3.0, the backup will be faster, but as you have OS X 10.5.8, I suspect it's connected through USB 2.0
iomega.com directs the user to go to the apple icon and select Control Panels but that option doesn't appear @ my MacBook Pro.
First off the software and advice your likely getting from iomega is to use THEIR backup software, which isn't 100% compatible with Mac's or Apple.
Also the drive is likely formatted to work with their backup software as well which TimeMachine will ignore as it thinks it's a drive it can't make a TM drive with.
So you need to ignore and quit their program if it's open and head to Disk Utility in your Applications/Utilities folder.
Select the external drive on the left and click the Erase Tab, then click Erase. Disk Utility will format the drive the default GUID and OS X Journaled that TimeMachine can use. Formatting a drive erases all data on it, so if you only have one copy of data on that drive you want to keep, you will need to copy it back to the Mac's drive.
As you should know, dragging and dropping copying makes a copy, it leaves the original file on the original drive.
Next to set up Time Machine, open System Preferences > Time Machine > Select Disk
Also if your really on 10.5.8 on a Intel processor Mac, then you need to upgrade OS X as yours is insecure and no longer getting security attention.
There are other backup methods and you should NOT rely solely on any one backup system, especially something complicated, hard to restore if it's corrupted, can't be used with a PC and holds your files hostage like TimeMachine does.
Since a TM drive is connected more often, it's subjected to shock damage, theft and malware a lot more than a less often updated bootable clone. Also TM drives are not bootable to use the machine like clones are.
So learn more about your backup options to have hardware and software redundancy with your precious data.