open the package containing the mesh you want in the generic browser. Right click on the mesh you want to copy and select duplicate. You will come to this screen:

select your maps package and click OK. Now open your maps package and double click on the mesh you just copied to open the static mesh editor:

fully expand everything under the LODInfo tab. Circled in pink above is the line you are looking for. You will have to copy the material used on the mesh into your maps package also. This is done the same way as you copied the mesh but with a few more instructions:

each material is made up of 1 or more textures listed as texture sample in the material editor:

you will have to copy all of those textures into your maps package also by using "duplicate". When you have the material and all of its textures in your maps package you will have to reassign the texture in your maps package to the material in your maps package using the material editor or you will get missing package errors.