I've been away and won't be back for another week. After that I can check the Windows version of WireShare to check what might be happening.

There's a small box you enter the single IP address into. It only accepts a single standard numerical IP address, not any address that includes the (mind block - reverse dns lookup) alternative name for the address with alphabetical letters. After typing or pasting in the address you press add or press enter on your keyboard and the number should then be seen in the list below. From memory I use both enter and add but I can't check at moment.
If you try to add a number with illegal characters (such as alphabetical letters) then it will fail to add.
e.g.:
Legal: 7.100.73.10
or 7.100.73.10/32

Illegal: rus.7.100.cable.73.10
or 7.100.73.10-32

Edit: Come to think of it, I think WireShare might have some limits on how large a block range can be. Or perhaps I'm confusing with LW 4. But you are unlikely to want to block such a massive range anyway.