Hi anshbat,
the exact syntax of left or right is as follows
Left([Message], 3) = ""FOO""
or
Right([Message], 3) = ""FOO""
and they output True or False.
If you have a decision stage (the diamond stage) and you put one of these statements in, it should work for you then.
Left([Data1],3) = ""CON""
Should be the context you are looking for. You don't need to write 'if' in the stage either, just the text above. Then connect the decision stage to the Yes/No stages you want them connected to.