Let's put the floor on your map.
1 - First you have a map structure, with walls.
2 - Now cliq in "Block Tool" [Shift + B].
3- Select the portion that will put the floor. screen (x / y). screen (x / z) will select the last row.
4 - Select the texture by clicking next "Browse" and select the texture, then go to the part you selected the location and click the right button, then "Create Object".
5 - Ready your floor is created.
Any doubt post in the topic, Thanks.