You can do that only if you have permissions to view and edit accounts. By default only Administrator can do that.

To add Contact to Account, do the following:

1. Click on Accounts in navigation bar and then again on Account in the drop down menu. List of Accounts will appear.
2. Click on Account you want to add Contact to.
3. Under Account members, click on look up button and add contact by clicking + sign in front of his name.
4. Click Save to save Account