Get a terminal, go to the desired location (cd /usr/lib/ladspa) and do a ls -l. That will give you a list of all files including the permissions.
The first 3 bins are for owner read,write,execute (well, starting from the second one on, actually). The next three are the same for the group and the last three for everyone else. So basically, what you want is to have something like this -rw-r–r-- which would mean that it is readable by everyone.
It means that root may read and write and the rest may only read from that folder.
If it only would be -rw------- you would probably needed a “sudo chmod 6667” or something similar command to make this folder available for Renoise to access.
But don’t these plugins need eXecute permissions as well?