I had an opportunity to trained with a multinational company sometimes ago, during the training i attended an interview for the position i was acting, but before the result came i got an offer from a private organization with 3months probation. Now i am just 1month old my former trainer called me for the position i left. I did not know what to tell my current employer, i have sent an acceptance letter to the company asking me to resume by September, 2014. Kindly help me sort things out.
It doesn't matter who's hiring you. That's non of your current employer's business.
You quit, tell them that your current position is not meeting your expectations (obviously, otherwise you wouldn't be considering leaving) and you fulfill your notice time. They'll most likely be thankful towards you for not wasting their time.
You should, however, make sure that you've signed a contract with your new employer before burning all bridges. It would really suck if you'd quit and all you had was a verbal agreement with your new employer, who then, out of nowhere, could decide to not re-hire you (shit really does happen).