वानर monkey, कृतार्थेन by a grateful one, रामस्य Rama's, कृतम् task, इच्छता intending, त्वया by you, सीतायाः Sita's, मार्ग in search, यत्नः attempt, ननु नाम कर्तव्यः it should be taken up indeed.

'O monkey, if you are grateful to Rama you should wish to search for Sita. Indeed, you should make efforts to take it up.