The short answer is download the 64-bit version. Most operating systems are 64-bit these days and a 64-bit program can access all the memory in a machine, whereas a 32-bit program can only use a maximum of 2 GB. If the 64-bit version will not run you probably have an older 32-bit machine and download the 32-bit version of 010 Editor.
For the long answer, the following steps can be taken on different platforms to verify if you are running a 64-bit or 32-bit OS:
- Windows 7 - Press the Win+Pause/Break key on your keyboard (alternately click the Windows Start button, then right-click on 'Computer' and select 'Properties'). Under the 'System' area look for 'System type' which should read '64-bit Operating System' or '32-bit Operating System'.
- Windows 8 - Press the Win+Pause/Break key on your keyboard (alternately go to the Control Panel, click 'System and Security' and then click 'System'). Under the 'System' area look for 'System type' which should read '64-bit Operating System' or '32-bit Operating System'.
- Mac OS X - Click the Apple icon in the upper-left menu bar, select 'About this Mac' and then click the 'More Info' button. Under 'Hardware' locate the 'Processor Name'. If the 'Processor Name' is 'Intel Core Solo' or 'Intel Core Duo' you have a 32-bit processor, otherwise you have a 64-bit processor.
- Linux - Go to a terminal and run 'uname -m'. This command will display 'x86_64' if you are running a 64-bit OS, otherwise you are running a 32-bit OS.
A 32-bit program can be run on a 64-bit OS but note that Linux may require additional libraries to be installed to do this.