To download from Rapidshare should not be any problem provide if you are a premium account user. If you are a free user as me, you will have to wait for a countdown before you can start downloading your first file. Then wait until your first file been fully downloaded, only then you can start to download your next file, of course you have to go through the countdown again.
Jdownloader is just nice downloader that can make our life a bit easier, all you need to do is add in the URL of the files you want to download, Jdownloader will help you to download it one by one. You can go to enjoy your coffee now, Jdownloader will do the dirty countdown waiting for you.
Jdownloader needs no installation, you just download the zip file and extract it to a folder of your choice, and start using it. If your computer is not installed with the Java Runtime, you will need to download and install it first, because Jdownloader if an open source application writing completely in Java, so it is plate independent. You can run Jdownloader in Windows, Mac OS, or Linux.