Sigh, I had this problem.
Once you decompile the .mdl file, go to IMPORT.
Then go to the folder where the .mdl file was.
THere will be a lot of .smd files instead of one .mdl file.
One .smd file has the same name as the original .mdl file.
If the .mdl file was called 'Goku' then you need to find the .smd file named 'Goku' and open it.
It is that simple.