It should be possible to work with both .mdl (3D models) and .act files (animations) in Animation:Master, the very same program that was used these files in first place. Unfortunately, we do not know about any other software that can work or convert these two files between one another.
Convert Animation Master 3D model to Animation:Master animation file.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert mdl to act on Windows
Program for creating 3D pictures and animations
Convert from mdl file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.