Why is it Chrno? and not Chrono? I found the spell of the title a bit weird and wondered if there was any significance to the spelling of Chrno.

The ultimate answer to this question is: because the original author chose so. In other words, it's his work, so he can use whatever spelling he sees fit.

Significance? Nobody BUT the author knows, unless he (or she, or whatever; I don't who the author is) reveals it at some point in his work. If he did, then I'm sure it's in the manga (and will be in the anime, inevitably). If not, then, well, use your imagination. Better yet, just accept it as it is and enjoy it for what it's worth.

name of main char is chrno don,t ask me why is name ain,t chrono thats just chosen by the makers of the anime

It might have something to do with the Chrono Crusade game out and others with the Chrono name, I don't know if any more out though. That could be a guess.