If I don't know the application I only know the Software requirements, on Software requirement basis I need to write the test cases in Selenium IDE. So how can I write the Test cases on Software requirement basis? Can anyone help me?
You can't. Your can write the test conditions, and to some extend the test specification. But the test implementation depends on the implementation of the HTML, since no concrete interface seems to be given. http://www.software-tester.ch/PDF-Files/istqb_glossary_of_testing_terms_2.2.pdf
It depends on how good the software requirements are written. If there are mock ups with clear process flow of the application you can try to write the tests getting the ID's of the objects you'll be interacting with from a file or excel sheet.
Of course it's not normal / advisable to do that, but there's always a way...