In the resource-bay, there are different tabs for the different resources. There should be a second tab for metal, which then is for the lander hull.
Or you pay an alien facility with repair capabilities to repair the lander hull. I personally use resources in the ship except minerals only as a last resort and instead recommend selling them to aliens which pay more (some really want some types of resources and pay more than average).

so how do you build a new lander if you blew up your old one?

Originally posted by Demiurge:

so how do you build a new lander if you blew up your old one?

You should be contacted by aliens, who have a production facility nearby, with the necessary information(blueprint, location).