Download installer from here. For Linux/Mac users, you need to run it via wine. Read this guide to setup properly.
How to use?
Install the downloader (of course!).
Run the downloader.
Copy mega links. The downloader will automatically grab the copied mega links
Tick files to add to queue.
Click "Start" to start downloading.
If you still get error when running the downloader, try repair .NET Framework with this tool: http://www.microsoft.com/en-us/download/details.aspx?id=30135.
If it doesn't work, try reinstall the whole NET Framework if it still giving error.