If this is causing you so much trouble, then the easiest thing to do is not try to run the update over the internet and do a local install, by downloading the stuff you need and using the Line 6 Updater Utility. First you will need the HX Edit v2.92 software, then the Helix LT v 2.92 Flash Memory Bundle.
O.K. go to the "DOWNLOADS" menu at the top of this page and from "-ALL HARDWARE-" select Helix LT, then leave the "-ALL SOFTWARE-" as is, now select Mac OS X from under the "-ALL OS-" tab. from the next page you can select Helix/HX Firmware 2.92 Flash Memory, then below that you will need the HX Edit 2.92. Now scroll further down the page a grab Line 6 Updater version 1.9. Download all these bits to your Mac( they need to be installed in the Line 6 folder inside your Applications Folder). If as you say this a new Helix LT , unless you have made some presets that you wish to keep then they need to be backed up using HX Edit prior to updating the Firmware. If everything if fine then proceed to next step. Ensure that you have your LT connected to the Mac by USB cable and launch the Updater Utility and select "Offline Mode". Your LT and current firmware version should show in the window, click on the dark green coloured band and you should now have the option to select a local file - point it at the Flash Memory file you down loaded and let it get on with the update.
Hope this helps/makes sense