I need to connect my bluetooth keyboard on my mac, but it doesn't have bluetooth installed

Possibly this if you have a spare USB port open...

http://eshop.macsales.com/item/IOGEAR/GBU321/

No eMacs had Bluetooth installed as a standard feature. There was a separate Bluetooth module that enabled that function that was an extra-cost option. I don't know if you can at it to an existing computer. eBay shows some modules but the seller does not make it clear if they fit work in an eMac.

What kind of keyboard? There are wireless keyboards that come with a BT receiver that plugs into an open USB port. If you get one, make sure it works with OS 10.4. This wone works back to 10.3.9 according to Iogear's web site:

http://eshop.macsales.com/item/IOGEAR/GBU321/

Ya beat me, BD! Was off looking for my virtual stash of Apple service manuals to find the part number. Struck out for now.

If your eMac us a 1.0 Ghz CPU or slower there was no Bluetooth available for these models.

You can do a search for USB style Bluetooth plugin modules, but they have to be Mac compatible.

I do not think you can use any ole' USB Bluetooth plugin device.

In the case you have explained you will have to do some shopping around. I am sure there is something out there that is available and will work with a Mac. The key here though is it must have the software available with the Keyboard or Mouse to insure that it will work on your older model Mac. Normally all you need to do is follow there instructions to a tee to make them work. I have 2 different items that work trouble free on 2 different Mac's. One other thing to keep in mind is you may have some trouble tracking down what you are looking for.

Here may very well be a place to start.

https://www.macsales.com/

CHEERS

AND GOOD LUCK ON YOUR QUEST.

DON