[Help] Problem of running selenium Testcase[Java Junit] with Selenium RC

Hi,
I have a problem of running a Testcase of Junit(java) with the Selenium RC Server.
I am using Eclipse IDE. Selenium Documents says that we need to run the selenium RC as follows:

java -jar selenium-server.jar -userExtensions user-extensions.js in the command prompt.

From where do i have to take this "user-extensions.js" file?. Please clarify more about this User Extension. I have already read about User Extension from Selenium Documents.

I was taken a "user-extensions.js" file from smartgwt2.4/selenium/user-extension.js and i run the file along with the selenium-server.jar in the command prompt.
I got the following error as follows: [please help me to solve this issue, its emergency]

com.thoughtworks.selenium.SeleniumException: ERROR: Element scLocator=//DynamicForm[ID="isc_DynamicForm_0"]/item[name=fName||title=Name||value=nobel||index=0||Class=TextItem]/element not found at com.thoughtworks.selenium.HttpCommandProcessor.doCommand(HttpCommandProcessor.java:73)
at com.thoughtworks.selenium.DefaultSelenium.type(DefaultSelenium.java:178)
at com.example.helloworld.client.HelloWorldTest.testHelloWorld(HelloWorldTest.java:19)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at junit.framework.TestCase.runTest(TestCase.java:168)
at junit.framework.TestCase.runBare(TestCase.java:134)
at com.thoughtworks.selenium.SeleneseTestCase.runBare(SeleneseTestCase.java:71)
at junit.framework.TestResult$1.protect(TestResult.java:110)
at junit.framework.TestResult.runProtected(TestResult.java:128)
at junit.framework.TestResult.run(TestResult.java:113)
at junit.framework.TestCase.run(TestCase.java:124)
at junit.framework.TestSuite.runTest(TestSuite.java:232)
at junit.framework.TestSuite.run(TestSuite.java:227)
at org.junit.internal.runners.JUnit38ClassRunner.run(JUnit38ClassRunner.java:83)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:46)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)