A lo mejor
I don't know about the Spanishdict gadget. The translation is: perhaps, maybe, as likely as not
To me an accurate translation of "A lo mejor" would be "At best". There are other words in Spanish that work better to say maybe like "talvez" and better to say perhaps like "quizas".
Your question: Is it possible to look up phrases in this "spanishdict" gadget?
Yes. Across the top of the home page are various tabs, one of which is 'translation'. There you can translate phrases.
Renae, I agree with you. But then I kind of started thinking about what "probably" is actually saying (or, what you could replace it with in English).
Just for a scenario...
P1: "Is he going to be late to the party?" P2: "Probably, yea."
P2's answer could (not saying it's 100% accurate or similar) be replaced with something like, "That's my best guess." or "It's best to assume that."
Sometimes we have to translate our own language first to make sense of a phrase in Spanish.