you have to have to use the filename of the template file, not only the extension.
So Solid Edge is able to use the desired file as the template for the imported geometry.
try it like this:
documents.OpenWithTemplate("C:\temp\ASM.jt", "C:\Program Files\Solid Edge ST9\Template\ISO Metric\iso metric assembly.asm")
Off course you have to change the path according to your installation...
You can use the GetTemplateFileName() method to find template files or search you directories directly.