Auto select in a drop down box

Try this...

Sub Demo()
MyList = Range("A1").Validation.Formula1
MyArray = Split(MyList, ",")
MyChoice = Int((UBound(MyArray) + 1) * Rnd())
Range("A1") = MyArray(MyChoice)
End Sub

assuming validated cell is A1 and you have no commas in your possible validated values.

Its a macro, not a formula.

Hit alt F11, right click on the explorer tree to add a module and paste the code in there.

Then back on the worksheet, hit tools-macros-run to run the macro.