When you import a OBJ, Cubik also imports MTL and PNG(s).
To proceed, Cubik needs to know where to look for MTL and PNG(s) files. Most of the time, MTL and PNG files are aside OBJ file or in a folder aside with valid paths indicated in the OBJ file (on top) but some 3D software do not create OBJ files with valid relative paths...
Please, post your OBJ+MTL+PNG files (in a ZIP) or open your OBJ file (with a text editor) and verify the line beginning with "mtllib" followed by the name of the MTL file. If this line is missing or name or path is invalid, Cubik can't import MTL.
Several times, the problem can also happen because texture paths are invalid in the MTL file itself. To be sure that texture paths are valid, open MTL file (with text editor) and look for lines beginning with "map_Ka" and "map_Kd". Both should be followed by your texture name. MTL file can contain several lines like that. Be sure to verify validity of each one.