Frontier can be downloaded from the Frontier Download Page, payment success page after payment is completed, or trial success page after customer details are entered.
To install Frontier you must be running Rhino 7 on a Windows based machine. Detailed step by step instructions can be found here.
Yes, you can create your own unit cell geometry but only using the Grasshopper toolbox. The key is to create a perfectly symmetric shape, whether a mesh or a group of curves. Please see "Mapping a Custom Unit Cell" video under the Techniques video section for more information.
If your voxels are not matching your input surfaces, it is likely that your input surfaces are seen by Rhino as "Trimmed Surfaces" (learn more here). The resolve the problem, rebuild your surfaces as "Untrimmed". Please see the "Surface Input Preparation (Trimmed vs. Untrimmed)" video under the Techniques section for more information.