Welcome Cheirhandrea!
There are easily hundreds of variants of the iMac dating back to 1998. Many of the pre-2006 iMacs this forum covers cannot support an Apple wireles keyboard. Because this model has been in production for so long, it is important for you to let us know as much about your computer as possible so someone won't give you information that may not be appropriate to your iMac variant.
So we need to know:
1) Your processor type and Mac OSX version, both available by doing "About this Mac" from your Apple menu at the left end of the menu bar:
2) The make and model of the keyboard.
I asked for your Mac OS version because "iOS" as currently shown in your equipment profile cannot run on any Mac computer, only Apple's mobile devices. Once you get the OS version from "About this Mac," please change your profile to avoid further confusion and get help faster without the delay of someone having to ask for clarification before they can start helping.