Who wants a FizzBuzz?
From discussion elsewhere I know you don't want to change the basic implementation, so I'll confine my comments to idiom, style and tweaks.

I would replace the for loop with a foreach

Range // addon reference required in newer versions
1 to(100) foreach(i, toBuzzWord(i) println)


The Range protocol used to be in the core library but is now an add-on, hence the reference needed before you can use it.  That said, it's a more idiomatic, expressive and robust way to deal with simple ranges.  for loops are even more fragile in Io than in C-like languages, given the optional parameter and Io's blithe tolerance of extra parameters.

I don't like chained if-then-elsif in any language, even when it's a function and not a keyword.  One alternative would be to use return

method(n,
   if (n % 15 == 0, return "FizzBuzz")
   if (n % 5 == 0, return "Fizz")
   if (n % 3 == 0, return "Buzz")
   return n
)


Multiple exit points from a function are much debated and criticised, but


Io has no switch or case function and the community doesn't want one.
lists (using a numeric index lookup) or maps (using a string key) are sometimes used for very simple switch-style control flows but that doesn't work here.


In purist OO style, Io developers prefer to use (no surprise here) polymorphism.  Send a message and let the inheritance chain pick the appropriate action.

Polymorphism is really overkill for the simple fizzBuzz implementation you have chosen.  It might be useful for a more efficient solution.