Insert the following in your collectd.conf:

<Plugin python>
    ModulePath "/path/to/module/dir" # Not needed if installed via pip or package
    LogTraces false
    Interactive false
    Import "collectd_transmission"
    <Module collectd_transmission>
        username "myuser" # Required
        password "mypass" # Required
        address "http://localhost:9091/transmission/rpc" # Optional, defaults to "http://localhost:9091/transmission/rpc"
        timeout "5" # Optional, defaults to 5

modified accordingly to your needs. Restart collectd and you are done.

sudo service collectd restart

There should be RRDs for transmission under collectd’s data directory. Most probably that is /var/lib/collectd/rrd/<hostname>/transmission/