first of all
you need to insert block command I send an example...
then you specified the coords. of the where the block inserted.
along a polyline I suggest get coordinate method.
if you describe your problem with details, I can help with further imformation.
here is insert block example which already existed in acad
sub is add block to (0,0,0) coordinate, you can modfiy to polyline coordinates..
it is fairly easy to do mathematically. Here is one example. It is maybe little complex but will give you directions how to do it. There is a way with SendCommand() method but I don't know how it works. Look in Help Menu there is plenty of examples