Macros (%name%) aren't expanded in step script code. Change the code in the script event to
Function vbld_StepDone()
fname = Application.ExpandMacrosAndScript("%PROJDIR%\F6Tes TDotNetPort.root\F6TesTDotNetPort\%BUILD_HISTORY_L OG%")
If vbld_FSO.FileExists(fname) Then
Builder.LogMessage vbld_GetFileContents(fname)
End If
End Function
See
http://www.visualbuild.com/Manual/?scripteditor.htm for more details.