Are you tring to open a PLC5 file for editing? If so, then you need RSLogix 5.
If you are looking to convert the code to ControlLogix, then you could "Translate" the PLC5 program to ControlLogix. Look under the Tools tab, then Translate PLC5/SLC.
It's pretty straightforward. In Logix5 save your file as a .PC5 file. Then do Tools-> Database-> ASCII Export and ensure that under the AB6200 tab you're exporting the Address/Symbol as a .txt file
In Logix5000 do Tools->Translate PLC/SLC and fill in the blanks. Don't expect to get 100% done for you, there'll almost certainly be a list of exceptions to go through. Then in Logix5000 do File-> Open but remember that you're looking for a .L5K file rather than .ACD. You'll be asked if you want to import, you won't get anywhere by saying no!
But even then it won't be a PLC 5/80 program anymore. It'll be a program for a ControlLogix. And the addresses will be all messed up. And some instructions won't convert nicely -- or won't look like they used to in the 5/80.