अन्वयः

तस्य शैलस्य that mountain's, दक्ष्णिपश्चिमाम् southwest side, कोटिम् end of, आसेदुः reached, तेषाम् for them, तत्रैव while being there, वसताम् while they stayed, सः कालः that time, व्यत्यवर्तत slipped.

M N Dutt

And they came to the south-western summit of the inountain. And when they sojourned there, their (appointed) term had not expired.

Summary

They reached the end of the southwest side of the mountain(Vindhya). While they were still there, the time stipulated for them (to find Sita) had slipped.

पदच्छेदः

तेषां	तद् (६.३)
तत्रैव	तत्र (अव्ययः)–एव (अव्ययः)
वसतां	वसत् (√वस् + शतृ, ६.३)
स	तद् (१.१)
कालो	काल (१.१)
व्यत्यवर्तत	व्यत्यवर्तत (√व्यति-वृत् लङ् प्र.पु. एक.)

छन्दः

अनुष्टुप् [८]

छन्दोविश्लेषणम्

१	२	३	४
ते	षां	त	त्रै
व	व	स	तां
स	का	लो	व्य
त्य	व	र्त	त