In order to make nymead load your plugin, you have 2 options. Either install it into the system or make nymead load it from your plugin build directory.
To install the plugin into the system, run
$ make install
To make nymead load the plugin from the plugins build directory, run nymead with an environment variable exported
$ NYMEA_PLUGINS_PATH=/path/to/plugin/ nymead -n
In order to easier debug things, it is advised to enable debug output for the device manager and your plugin. E.g.
$ NYMEA_PLUGINS_PATH=/path/to/plugin nymead -n -d DeviceManager -d YourPlugin