FYI - this worked for me

[matygrosman]

This question is in reference to Atlassian Documentation: Trust A Server's Host Key when No Button is Available to Trust It

This solution below worked for me. Had like 20 instances of plink.exe running in task manager. Once I killed all of them and tried again, the buttons came up. 

https://jira.atlassian.com/browse/SRCTREEWIN-140

[ackvf]

Also adding a host using plink works

http://stackoverflow.com/a/12448986/985454

`plink.exe` is located in `C:\Program Files (x86)\Atlassian\SourceTree\tools\putty`

usage: `plink.exe github.com`, then confirm by `y`