This is a bit tricky with Alice's drag and drop interface.
You first have to set up the IF statement with just true or false.
Then drag the functional test into the statement. The only functions this will work for are those which give true/false answers. (Amazing??

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If the function you choose is what Alice calls a "boolean logic" function, you will have a straightforward choice of variables.
If, on the other hand, you choose a "math" function, you first have to set it up with numbers. You can then drag and drop variables of most types (maybe of all types - just haven't tried every one) as needed - making sure that the variables being compared are of the same type.
A very quick example is attached. Change the default values of the variables to verify for yourself that the IF statements are working.