There are common defaults set in the plugin, here is an explanation of those you should change to match your setup.
Open the plugin's entry in OctoPrint's settings interface, and you should be presented with this view:
WS281x LED Status settings
Click on the 'LED Strip Settings' button in the top right. The key settings you should edit are listed below:
Strip Type Select the type of strip you have connected to your Pi. Note: This may not be exactly what you were sold, sometimes they send variations. If you have issues, please try similar strip types here.
Number of LEDs This should be easy, count the number of LEDs you have connected and enter the right number!
Max Brightness This setting controls the maximum brightness that your LEDs should reach.