Firstly are you sure that's the path to the file? ie its a file called filename1.ppt directly on the C drive.
secondly the code you quote will only insert one slide (assuming it's the right path!)
syntax is insertfrom file > the (correct) path&filename > The slide number you want to insert after (so 1 is probably good)>The slide number to start inserting from (1 is still good) > THE NUMBER OF SIDES TO INSERT 1 is probably bad!. If you simply omit this last value all slides will be inserted.
So (assuming the path is correct :
ActivePresentation.Slides.InsertFromFile_ "c:\filename1.ppt", 1, 1
For the next file count the slides and start inserting after that number:
ActivePresentation.Slides.InsertFromFile "the proper path", ActivePresentation.Slides.Count, 1