Hi,
1. Have you not noticed the 'Help' menu? In German this will be 'Hilfe'. From it you can select the Quick Start option which will display a document describing basic usage of Yummy FTP. The "I" in the toolbar is for getting Info about the item(s) you have selected in the listings - it doesn't respond because you haven't selected any items.
2. I think you are referring to the Keychain. This is a standard part of the Mac OS X operating system which allows you to store all your passwords securely in one place.
3. By default, all the toolbar icons do not have any text labels because the icons make their functions self-explanatory. However, you can see the explanatory text if you wish - control-click on the toolbar and select 'Icon & Text' (In German, 'Symbol & Text')
Maybe everything will be clear to you once you read the Quick Start. Unfortunately, it is in English, but it should still be useful.
Thanks,
Jason