I suggest, first back up your current Express Accounts data before attempting the re-install. ('Tools' menu -> 'Back up Data...')
To re-install on Mac:
Download the current version, and double-click on the installer (.dmg file), follow the instructions, and finally drag the 'ExpressAccounts' icon to 'Applications' folder. If prompted, replace the old ExpressAccount item with the same name.
Also, note that you may have to re-purchase the license, because it only allows free upgrades for up to 3 months from the last purchase date.