I had the same problem. All you have to do is start the 18.1 install again (you don't need to uninstall what you already have, this is not going to change anything). The install unpacks the necessary files and will give you 2 options for the location, the recommendend one (first in the list) is a temp location that will be deleted when the install is done (which I suspect is what you did first time around), the second is to a directory that won't be deleted, pick the second. Allow the process to unpack all the files taking note of the location, and then when it askes to continue to the next step press cancel.
Now when you apply the patch and it says that it can't find the .msi file, browse to the location of the unpacked files and it will be their.