The drawback to the double-click approach is that it only installs an add-in temporarily, until you close Excel. When that happens, all add-ins are unloaded from memory. But, unless you plan to always launch Excel by double-clicking the ViXL3D.xll file, Excel will not remember to reload it on subsequent launches.
Should that happen, any spreadsheets you might open that used one or more ViXL-3D functions will report an error when any function-related cell is recalculated. Should that ever occur, close the spreadsheet without resaving and restart Excel by double-clicking the ViXL3D.xll file.