You don't happen to have any DD diskettes (the ones with only the write-protect hole)? If not, get a regular HD diskette and tape over the hole on the opposite side of the write-protect hole. The HD diskette will most likely not work well very long as a DD diskette, but it will suffice to transfer a file.
When you have fixed a disk, doubleclick on Storage/Dosdrivers/PC0 on your workbench-partition and you will be able to read/write pc-formatted disks.
Under most OS:es on a PC, it is a bit tricky to format a DD diskette, so just insert it in the Amigas drive and doubleclick on System/Format on your workbench-partition. In that program, select PC0: and format it.
You can fit 720kB on a pc-formatted DD-disk and as the AmiTCP 3.0b2 archive only is 686kB, there will be no problems fitting it.
/Patrik