LyX ⇒ Document Class not available

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I'm at my wit's end. I tried for hours now to get LyX up and running. I have to use LyX for a scientific research report for my university. I'm using Lyx 2.0 and MiKTeX 2.9 on a Windows 7 64Bit Laptop. First I install MiKTeX and then LyX, but every time I'm trying to use LyX with the files provided for my work I get the error that the document class book (koma-script) is not available and that scrbook.cls is not available. However, if I look into the MiKTeX Package Manager koma-script is clearly installed and scrbook.cls is in the MiKTeX folder structure where it should be. Does anyone have any Idea what's causing my error or how to correct this? I'm grateful for any help.