Custom codec & Codec Pack available in Google Play are different things. Don't install Google Play codec packs until or unless MX Player ask you to install.
If you are looking for Custom codecs, here you have to go.
1. Check which version you need by going to Settings/Decoder/General
in the Custom Codec tab it will tell you which one is needed (x86,tegra2,tegra3,neon).
Note: skip step 1 if your using aio (all in one) version
2. Download the one you need (No need to unzip it)
3. Go back to Settings/Decoder/General
and tap the Custom Codec tab then select the version you downloaded. DONE!
Thanks for the reply. I thought I was following these instructions. Perhaps I'm not fully understanding the sequence or how to do it. When I go to Settings/Decoder/General, it says "Use ARMv7 NEON type custom codec". Since you say the Google Play codec is incorrect, where do I download the actual codec I need and how do I install it? Step by step please.