I have an IMAC from 2008, version 10.6.8 with 4GB of memory

I have an IMAC from 2008, version 10.6.8 with 4GB of memory, and are trying to upgrade to Maverick?, but ending up with "Waiting ..." I have tried over the nigh ... but nothing happend? can someone tell me what I´m doing wrong?

hans4711