Synchronizing from zero takes a very long time, I would expect 1-2 days on an ADSL connection.
If you are properly connected to the Gridcoin network you will see the number of "Blocks" increase every few minutes on the wallet summary page. The block count is now over 1,200,000.

Rebuilding the chain will reset synchronization from zero and start again, this feature can fix problems if your copy of the block chain gets corrupted. Alternatively the option to Download Blocks will open a new window that downloads and installs a snapshot of the block chain which will speed up the synchronization process.

Having a recent version of the Gridcoin client is important, installer packages for the latest version are available at https://gridcoin.us/ near the bottom of the page.

If you already have the latest client and can't connect to the network then you should check to see if your firewall or virus protection software might be blocking the program.