Although we could work out the scripting to call actions directly, you can normally accomplish what you want through Action Groups or, if creating a control page, actions on those.
I apologize if you already know this, but since you are new to Indigo, I'll walk you through creating a Roku action.
* Create a new Action Group
* Add an action Device -> Roku Network Remote Control -> Send button-pressed command to Roku.
This should pop up a dialog of action options and you can select which device and then edit the action settings to select which button. NOTE: you will need to make sure you defined the Roku as an Indigo device first. Of course, can string those together with several actions in a row if needed.
If you really need scripting, I would suggest starting with the Wiki, it has a section on scripting plugin actions. Once familiar with that we could get the values required for various Roku actions.
https://wiki.indigodomo.com/doku.php?id=indigo_7.5_documentation:plugin_scripting_tutorial