I know that to install an Unsigned application on phone, one need to select a option from setting "Allow Installation of non-Marked application", but this option is not available in my Samsung S3 phone.
How can I install an Unsinged Application ?
You cannot install unsigned applications.
The "non-Market application" or "unknown sources" setting only allows you to install signed APKs from other sources than the Play store.
For non-release testing purposes including tracing with DDMS, sign your APK with a debug certificate. Android IDEs do it for you automatically.
For release purposes, sign your APK with a release certificate you've created yourself.
Further reading: http://developer.android.com/tools/publishing/app-signing.html