Unlike running Selenium IDE scripts through WebDriver, You don't use the Selenium plugin for RemoteLaunch, instead you should use the JUnit plugin (see http://www.inflectra.com/SpiraTest/Downloads.aspx) that will let Selenium tests written in Java / JUnit report back to SpiraTest and then use the Command-Line plugin for RemoteLaunch to actually initiate the execution of the test scripts.
To have JUnit report back to SpiraTest, you simply use the JUnit listener that comes with our extension and use that in conjunction with the special @SpiraTestCase attribute on the Java class to denote which test case in SpiraTest the results should be associated with.
You can then run this JUnit from the command line using Java.exe or use an IDE such as Eclipse. If you are using Eclipse, make sure to execute as a Java application not a JUnit test, since the built-in JUnit runner for Eclipse doesn't support custom listeners (or not at time of writing)
Once you have the basic JUnit setup in place reporting back to SpiraTest, you can simply use the Command-Line plugin for RemoteLaunch to kick off the JUnit test harness. All of the reporting back to SpiraTest is usually left to the JUnit test fixtures themselves.