I must apologize for my answer.  I have been so focused on video these days, I missed the fact that you downloaded the audio cleaning lab, not the "video cleaning lab" they are both similar products but with different focus.  So I just downloaded the demo for the "Audio Cleaning Lab" product.  What you need to look for is similar to what I said before except for the last item on the list. 

c:\program files\Magix\ Audio_Cleaning_Lab_MX_Download_Version (or something similar)  that is the subdirectory where you will find the application to run the cleaning lab.  The file is named "CleaningLab" and is a application or .exe file.  If you don't have that sub directory under the path described, you probably only downloaded the installer, and do not have the application installed on your system.  If the program installed correctly, you should have had a desktop icon place on your system that looks like this

If you don't have either the sub folder or the icon, you need to find the original installer which is probably in your "Downloads" folder.  If you still can't find it, download it again, make sure you do a "save file" and see where the file goes.  You are going to have to run that file to actually install the product on your computer.  I hope this helps.  If you still have trouble, try sending me a personal message.

It seems you got two questions:

1.I have download cotent pack of Magix audio cleaning lab with Demo video how can i use this content pack and also I dont know where it is installed so pl help me,

Answer: As you did not mention what is your Operating system(OS)therefore, difficult to guess the download location but if you use windows computer you should got any one Xp, Vista or Windows 7. If you do not use any download manager for downloading then your downloaded contents pack should be inside the folder on C drive by the name Downloads. Click on My computer then click on Local disk (C drive Icon) it will open folders on C drive locate folder by the name DOWNLOADS and check inside.

If you still got problem then repeat the download steps. Go to Magix, go to download location as you did first time put cursor on the desired item you have downloaded before. Click on Download if it ask you where to save note it down, you will know the download path. You can check easily your saved contents following that path or continue download this time look where it is going to save. Open the folder double click on the contents file it will be installed easily.

2. Cleaning Lab software running perfectly without any error.

Answer: The software is running fine because you did not install contents pack yet and hope even after installation will work fine.

Note: if you know the name of a content pack file type that in Run command it will show you where contents pack is saved. Here is an example from Windows vista home premium. (My system is in Japanese)

Hope above answer will help you.