Do you mean an application, like "Notes" for iOS (*) or "Notes" for OS X, which is included in OS X 10.8 Mountain Lion?
If it is that, then you have to get OS X 10.8 Mountain Lion onto that iMac. It costs a whopping 19 USD though, thus you might be better off with a notepad.
To edit your thread title to a descriptive one, to get a knowledgable audience and help making the thread easier to find for people looking for the same information, just click on the
button on the bottom right of your original post and then click the