रामकारणात् for the cause of Rama, भर्तारम् lord of vanaras, यदि च उद्योजयिष्यामि if i motivate him, ससैन्यस्य of him along with his army, तस्य his, आगमनम् coming, व्यर्थम् useless, भविष्यति will become.

"If I get the lord of vanaras and his army for the cause of Rama (without talking to Sita), their arrival will be useless (if Sita would have given up her life by then).