I just used the Add-In tool to activate additional spreadsheet function. I
now have a function in place [=RANDBETWEEN(0.9,1.1)] but it isn't active,
i.e. the cell displays the number "1" and when I use the cell as a multiplier
it multiplies by one. In other words, the cell isn't functioning by giving me
random numbers between 0.9 and 1.1.

I suspect that there is some sort of "activation" key combination that I
have to hit to get the function to operate - but I don't know what it is.