What you probably have to do is to make your Windows drives visible in the emulation. This way you will be able to choose any file on your Windows harddrive from octamed (or any other Amiga program).
To do that, in the step where you hit F12 and select your octamed adf on the Quickstart page, go to Hard drives page too and make sure the Add PC drives at startup option is checked. After that, hit Reset button on the bottom of the window to reset the Amiga and start booting from the adf.
After it boots, you should see a couple of more icons beside the octamed one (representing the adf). The other ones are your PC hardrive partitions.
Now you should be able to browse these drives in octamed. I'm not sure how the octamed's "open file" dialog lookes like but look for "Drives" or "Volumes" button. The harddrive partitions should be designated by DHx (where x is a digit). The adf will be marked as DF0. These identifiers are equivalent to drive letters on PC (you know, A is a floppy, C - main partition, D and so on - other partitions and drives like CD-ROM).
