Okay the two pissed off Greek leaders have just embarked on a ship and they...
It means "and they placing the most valuble of (their) things" but why is itκαὶ τὰ πλείστου ἄξια ἐνθέμενοι
πλείστου which is in the genitive. Something like this would be closer to what I'd expect:
καὶ τὰ πλείστα τῶν ἄξια ἐνθέμενοι
Xenophon, Anabasis 1:4,7 wrote:
καὶ Ξενίας ὁ Ἀρκὰς στρατηγὸς καὶ Πασίων ὁ Μεγαρεὺς ἐμβάντες εἰς πλοῖον καὶ τὰ πλείστου ἄξια ἐνθέμενοι ἀπέπλευσαν, ὡς μὲν τοῖς πλείστοις ἐδόκουν φιλοτιμηθέντες ὅτι τοὺς στρατιώτας αὐτῶν τοὺς παρὰ Κλέαρχον ἀπελθόντας ὡς ἀπιόντας εἰς τὴν Ἑλλάδα πάλιν καὶ οὐ πρὸς βασιλέα εἴα Κῦρος τὸν Κλέαρχον ἔχειν.