Hi
You need to open the manifest.xsf file in visual studio not the sharepoint site.
manifest.xsf file is present in the solution file or you can extraxt it from the xsn.
Below are the steps to extract the manifest.xsf file from the ' .xsn' file.
1.Open your ' .xsn' file in infopath designer
2.Go to File menu
3.Select 'Save As Source File' option
4.Create a folder in local drive and browse to the reated folder
5.Click Ok
You will find manifest.xsf file in the folder.