I have yet to find the Insert/Overwrite toggle in UE/Mac at all. I wonder if that feature even exists, yet.
I have had success switching from PC to Mac, however, by doing the following.
First, use a PC full-function keyboard. PC's have better native keyboards because they provide several buttons that Mac native keyboards don't have such as Insert, Delete, Home & End. If you're accustomed to PC, you'll find it hard to live without those. PC keyboard functionality is fully supported by OS X, it being Unix and all. If you have a PC keyboard with multimedia, volume and sleep buttons, you'll find it particularly useful in OS X. The only thing you'll miss is the Eject key, but if you're a PC user, you're probably not very used to using that one, anyway.
The second trick will help you become more Mac-ish. Swap the Alt and Win-Start keys on the left side of your keyboard. Do this carefully. Most keyboards are built in such a way that you can simply pry the keys off (carefully) with a screwdriver or a knife. Pull gently from under the corner of the keys and they'll pop right off. Then, snap the Alt key back on where the Win-Start key was and vice-versa. After that, go into System Preferences->Keyboard->Modifier Keys and swap the Command and Option keys. Doing this will put the Win-Start key in the same location as the Command key on traditional Mac keyboards. Now your PC keyboard will be geometrically more similar to Macs. This is important because Mac people designed most of the keyboard shortcuts around the Command key, with the intention of you using your left thumb to perform key combos. It will be more ergonomically pleasing for you in the long run.
Third, overload the shortcut keys in UltraEdit with both Windows AND Mac key combos. I went into UltraEdit->Preferences->Key Mapping and set the main selection to Windows Keys. Then I went into each of the important commands and set both the Ctrl+ AND the Command+ key combos. For example, I set Save to both Ctrl+S and Command+S. I got creative for a few things such as Save All; the F12 key in Mac is taken by Dashboard, so I set my Save All to the 4-finger Shift+Ctrl+Command+S (think: "ALL shortcut keys + S = Save All").
Having both the classic Windows shortcuts and the traditional Mac shortcuts working simultaneously in UltraEdit has really helped me make the transition from PC to Mac. I only wish I could set the same shortcuts up in my other Mac apps.