Creating a mission via the "New mission" button was most likly disabled due to the way maps are loaded in Blockland, aka via the Add-ons Folder.
If you wish to create a new map, create a folder within Add-ons and call it Map_Namehere. Then right-Click on Map_Bedroom and drag and drop it ontop of the new folder and extract all the contents within it.
Once this is done rename the files to your new map name. Though at this point you probably still have no clue on how to open a .mis file and some other wanker can explain that.