Do you have a diff tool installed on your system?
In Mac, when you install Xcode you get a diff tool named FileMerge, simple but sufficient in most of the cases. You can try WinMerge for Windows, I’ve used it in the past and it’s pretty nice, and open source.
You can also try the P4 Merge tool from Perforce which is offered for free.
Both options can be selected as a Diff/Merge Tool in SourceTree.