I assume that you are referring to <a href=http://www.wizards.com/dndinsider/compendium/power.aspx?id=13742>Slimy Transmutation (level 1 wizard daily from the red box)
It's effect line reads:
As a toad, the target is dazed and the only actions it can take are to move its speed or shift.
As per the effect line of the power, a spell caster would not be able to cast most, if not all, their spells.
In general, the only limitation on a polymorphed creature taking actions is the explicit effects described by the polymorphing power. Wild Shape (druid class feature) is a good example here. It is a polymorph power, that has the following limitation on actions:
While you are in beast form, you can’t use weapon or implement attack powers that lack the beast form keyword, although you can sustain such powers.
Were you to have the wizard utility power Sleep, which is not an implement power, you would be allowed to use it while polymorphed.
