If this is COM object, you might have a look at examples in ThinBASIC/SampleScripts/COM.
I wanted to help you with the translation, but sadly I do not have the X10.ActiveHome on my PC installed. To instantiate it, you would need something like:
But as I said, I cannot test the rest, as I do not have this interface on my PC. The mentioned sample scripts should help you, I hope.
' -- Create object
DWord ActiveHomeObj = COM_CREATEOBJECT("X10.ActiveHome", RetVal)
' -- Call method (parameters should be supplied in form of Variant array)
Dim Params() As Variant
Params(1) = "sendplc"
Params(2) = "a1 on"
COM_CallMethod(ActiveHomeObj, "SendAction", 2, Params(1))
' -- Release it before script ends