Review the Application Performance Monitoring Guide. It is included in the TM ART documentation set. You'll find it on your TM ART Central by going to Help, Documentation. The second half of the document is all about using Silk Test based monitors in TM ART.
Note that Silk Test must be installed on all Execution Servers which will run Silk Test based monitors. There are also some Silk Test specific settings that must be made on the Execution Servers.
And lastly: Silk Test 15 is much newer than TM ART 4.2. I'm unsure that your version of Silk Test will work with your version of TM ART. You should discuss that with BMC Support.