This article shows how to create a TestCaseStep using a browser REST client
Endpoint:
https://rally1.rallydev.com/slm/webservice/v2.0/testcasestep/create
Payload example:
{"TestCaseStep":{"TestCase":"/testcase/<TEST_CASE_OID>","Input":"2x2","expectedresult":"4","stepindex":"0"}}
Here is a screenshot from Chrome's Advanced REST Client: