You can reach Lampi either from Myanmar or from Thailand.
From Myanmar you can take a local flight from Yangon to Kawthaung (duration approx. 3 hours, with stops in Dawei or Myeik.. Cost is approx. US$250-300).
At Kawthaung you will embark on your pre-booked tourist boat that will take you to Lampi.
From Thailand you can enter Myanmar at the Ranong border point in a long-tailed boat that will take you to Kawthaung (duration approx. 15 minutes, cost is approx. 250 Thai baht).