I recently purchased the Magix Music Maker Rock Edition. The software came with loops but I was wondering how I access the individual instruments to create my own loops.
I presume that you downloaded and installed all of the additional programs, instruments, etc. that are under Help, Update program/update functions, Download free synthesizer and sound packages? If not, do so.
Given the nature of your question, I presume that you are new to MMM. You should start by reading the manual to make sure that you understand what is going on. Read the Quick start chapter then look up recording - both audio and midi. Try recording something with each method.
To create your own loops, you have to record something, either audio or midi. For midi, click on the + button at the very left of a track and select an instrument. If you're using an external keyboard, make sure that it was plugged in, set up, and on before opening MMM. You can use the on-screen keyboard to play your own. Try playing the keys. Press on the record button and play something. Stop the recording. Listen to it.
For audio, you do not use one of the instruments from the program, you use an external instrument and record its output as an audio file.
Most of the Catooh loops that you have are short audio loops, specifically designed for looping. They are not all that simple to do properly.
If you have Magix loop designer under the Object synthesizers, then open it. It is mainly for drums. Read the manual to see how it works.