This is not a Jbpm question, it is a normal Java web application question, so you should look for the answer elswhere.
However, basically you cannot have the Java code sitting on your server, accessing a path on your client machine. This is obvious for security/design reasons etc.
What you could do is have your web browser "upload" the file, using a form. Make sure the enctype is 'multipart/form-data', then have a servlet on your server read that uploaded data, write it to a file on your server local disk, and then you can do whatever you want with it from there.
Try googling for "uploading a file using JSP" or something along those lines. There are a lot of places to find out how to "upload" a file using Java/JSP etc.