To get a perfect weld (chemical) job on ABS plastics (Mac cases, IIRC) you can use either ABS Cement, from anyplace that sells pipe, or use full strength MEK (Methyl Ethyl Keytone) for ABS, PVC or Acrylic (Plexi) and any other solvent weldable plastics. It's the active ingredient for all the glues for the plastics listed as well as styrene (Model Cement) and others.
USE IT OUTDOORS! KEEP THE BREEZE TO YOUR BACK! It's very flammable and the fumes are very toxic!
Clamp the crack together and apply with a brush to the back side and capillary action will flow it through the entire crack surface.
If you want to use it as Acrylic/Plexi Cement, as sold in Hobby Shops, just mix it 50/50 with clear acrylic/Plexi. You can dissolve more acrylic into it to get a gloppier solution to fill loose cracks better. I haven't tried it yet, but I'm saving Mac case scraps to try the same in proper MacBeige™ tones.