Since all Fortune Ladies are Spellcaster-Type monsters, you can Tribute Fortune Lady Light, or any other Fortune Lady for the effect of Magical Dimension ("Tribute 1 monster and Special Summon 1 Spellcaster-Type monster from your hand" is one action so Fortune Lady Light will not miss the timing, however, remember that Fortune Lady Light's effect will miss the timing if you choose to destroy a monster with Magical Dimension's additional effect). [quoted from wiki]
so now whats the issue? if i tribute lighty and choose not to destroy anything, i still get to activate effect?