'Show Difference' doesn't show anything

[DoCode]

git config:

[diff]
    <empty />
[difftool]
    prompt = false
[difftool "kdiff3"]
    path = C:/Program Files/TortoiseHg/kdiff3.exe
    prompt = false

[laurentkempe]

There is a bug described here #38 which will be fixed in next release
But in your case the problem is you configuration see the wiki for the correct configuration.