On PC use internet explorer. Chrome also works.
On Mac, get the newest version of Chrome and follow these steps:
Click the "Google Chrome" icon on your Mac's "Desktop" to open the Google Chrome Internet browser. The browser will open -- leave it open but move its window so you still have access to your Mac's Desktop.
Click your Mac's hard disk icon on the Desktop and navigate to the directory containing the MHT file you want to open. It is important that the name of the file ends with .mht. Sometimes after download, your computer may try to add .html or some other extension. Please manually delete that extension and than move on to the next step.
Click and drag the MHT's file name to anywhere within the Google Chrome browser's window. The MHT document will open within the browser.