Good news! I was having the same issue with Cura 3.2.1 on a non-retina 2012 MacBook Pro. I am running 10.13.5 Beta (17F35e). I just switched to the 3.3.0 BETA and it works. So this also works (for me) with the latest OSX Developer BETA.
Model Name: MacBook Pro
Model Identifier: MacBookPro9,1
Processor Name: Intel Core i7
Processor Speed: 2.7 GHz
Number of Processors: 1
Total Number of Cores: 4
L2 Cache (per Core): 256 KB
L3 Cache: 8 MB
Memory: 16 GB
Boot ROM Version: MBP91.00D9.B00
SMC Version (system): 2.1f175
Serial Number (system): C02J428LF1G4
Hardware UUID: B2DB5536-B555-52AC-9689-9C06C0901744
Intel HD Graphics 4000:
Chipset Model: Intel HD Graphics 4000
Type: GPU
Bus: Built-In
VRAM (Dynamic, Max): 1536 MB
Vendor: Intel
Device ID: 0x0166
Revision ID: 0x0009
Automatic Graphics Switching: Supported
gMux Version: 1.9.23
Metal: Supported, feature set macOS GPUFamily1 v3
Displays:
Color LCD:
Display Type: LCD
Resolution: 1440 x 900 (Widescreen eXtended Graphics Array Plus)
UI Looks like: 1440 x 900
Framebuffer Depth: 24-Bit Color (ARGB8888)
Main Display: Yes
Mirror: Off
Online: Yes
Rotation: Supported
Automatically Adjust Brightness: No
NVIDIA GeForce GT 650M:
Chipset Model: NVIDIA GeForce GT 650M
Type: GPU
Bus: PCIe
PCIe Lane Width: x8
VRAM (Dynamic, Max): 1024 MB
Vendor: NVIDIA (0x10de)
Device ID: 0x0fd5
Revision ID: 0x00a2
ROM Revision: 3682
Automatic Graphics Switching: Supported
gMux Version: 1.9.23
Metal: Supported, feature set macOS GPUFamily1 v3