Are you saying that you cannot find the keying effects Green Screen, Chroma Key in your version of Premiere Elements 12? If so, the major needed information is what computer operating system - Mac? Premiere Elements 12 Mac does not have these keying effects, and they are not available from Adobe. These are Windows Only effects.
Some suggest using instead the Videomerge effect in Premiere Elements Mac. Videomerge is not one of my preferred effects. I would suggest that you look at the Difference Matte under fx Effects Tab/Keying/ as an alternative.
Please review and consider, and then let me know if I have interpreted your question correctly and given you the information that you seek. If not, please give more details.
Any questions or need clarification on the above, please do not hesitate to ask.
Have you updated 12 to 12.1 yet using an opened project's Help Menu/Updates. If not, please do that.
If you need a step by step on how to replace a green background in a video/photo with transparency so that you can replace the background of the video/photo, please let us know.