What version of Premiere Elements is involved here?
Am I correct that you are talking about use of a file exported and saved to hard drive in Windows? We are not talking about using a project.prel created in Premiere Elements Windows and trying to open that in Premiere Elemens Mac? That is a not go.
AVCHD is a video compression type often designated MPEG4 AVC/H.264. What is the file extension (wrapper for AVCHD), .mp4, .mts, other?
Without more information about what you have, I am not sure that you need to convert what you have to anything else for import into Mac. What are the properties of what you have:
Frame Size: 1920 x 1080 or other
Frame Rate: 25 or 29.97/30 interlaced or progressive frames per second
Scan Type: Interlaced or Progressive
Pixel Aspect Ratio = Square Pixels (1.0) or other
File Extension: .mp4 or other
Based on that information and provided that you have the latest version of QuickTime installed on Windows and Mac...
and set Presets = NTSC DV 16:9 with customization to the required properties under the Advanced Button/Video Tab (video codec = H.264) and Audio Tab (audio codec = AAC or other).
If you have AVCHD.mp4, give that a try as is in your Premiere Elements Mac.
But more information is needed to fine tune.
To answer some of your questions.........
Program is Elements 11
File exported and saved to hard drive in windows
frame rate 29 frames/sec
Camera is a Canon Vixia HFR400
Can your Mac not Import the MTS files?
If it can, then that would be my recommendation, rather than any conversion, and especially as you are starting with compressed files to begin with.
For what you presented, I am not seeing a problem with importing the AVCHD.mp4 into Premiere Elements 11 Windows 7 or 8 64 bit or Mac 64 bit.
But, please confirm what you are starting with, that is, importing into Premiere Elements Windows in the first place (direct from the camera)...AVCHD.mts, AVCHD.mp4, AVCHD.m2ts. Here I am after the file extension not the video compression AVCHD which is often represented as MPEG4 AVC/H.264.