I downloaded a inventor 2008 iam file and when I open it in my software inv 2012,its all there in the browser-history tree- but theirs no model in the graphics screen,would anybody know why this is and have I done something wrong?
It's because all of the part files are not there. If the file was a .STEP or .IGES file the translator in inventor should have brought in all the appropriate data for the builds. If these were inventor files originally, then you'll need to download all of the associated files with the total assembly.
An iam file has all the data available for inventor to build an assembly (constraints, design accelerator parts etc) but not the parts themselves. The uploader should have made a zip or rar file with all the parts and the iam file. It is a common mistake made by new users.