I just did some research on your Casio WK-3800 and it does have audio outputs. You need a cable that will allow you to connect your keyboard to the line-input on your computer. Then you need to set your audio setting to receive the input. After connecting the cable, click on your volume control (speaker symbol - lower right corner) and choose you record setting to line-input. Make sure the jack on the computer you choose is the line-input NOT the mike input. THis should get you 99% there. Let me know when you get to the point and we will cover the Magix setup part.

There is light at the end of the tunnel, and hopefully that's not the train coming.  (OO).

Thank you Securityman.  Yes, I have R and L outputs in the back of the Casio.   I just finished returning from the store with two stereo cables, but have to return to get the dual-plug adapter to put in back of the PC.  I will chance it right now to see if the store is still open.  I'm really wanting to get this show on the road.  I shall return... do not disappear, lol.  *Shan runs to the store*

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Returned, store closed.  It will have to wait till tomorrow.  I will be so happy when I get this working.  ;)

 If you need to send an email to me2550525@yahoo.ca.

Hi Securityman,  I finally got the adapter.  I have both connections plugged in the back of the L and R out, and both cables are now connected to the dual-plug adapter (two mono cables, into an adapter converting to stereo).  Ok, now I'm going to go into the volume control.  ;)
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I think I got it. Yay!  I opened up the Magix, chose my sound card on it (in this case, it was "SoundMax Digital audio" and not my "Snowball mic"). 

The sound is now coming through.  Thank you very much Securityman !  I've saved your addy just in case something else pops.  ;)   Right now, you're my hero. 

I've got my headsets on now, and the sound is finally able to record. 
One track can be what I compose on the synth-keyboard, and then when I want to use my Snowball mic, I can just switch the "rec" settings on MM Pro 14 to my Snowball Mic.  I'm very grateful !  ;)

PC Color code 'plug in' is light blue.
PC Color code 'mic in' is pink.
PC Color code 'out' is lime green. 

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ETA:  For others, when the speaker symbol is opened, I went to:

1.  Options
2.  Chose 'Properties'
3.  In Properties, chose "Recording"
4.  Then I chose, "Wave Out Mix"  (others could have "Stereo Out Mix")
5.  I also chose "Microphone"  (for my Snowball Mic)

After I clicked 'ok', then the "Recording Control" came out.

I then placed a 'checkmark' in the little white box for "Wave Out Mix" (or Stereo Out Mix).

Then I could adjust the volume there as well, as on my Magix Rec.  

P.S.  I've just discovered that "Line In" on the "Recording Control" also works.

Volume can be adjusted either through the keyboard, the "Recording Control" and also via the Magix Mixer ... (or all three). 

 Let me know if you post any of your music :)