MrSID files display as black and white lines or as squares when added as data into ArcView 3.x.
ArcView 3.x only supports up to MrSID Generation 2; MrSID Generation 3 and newer do not display correctly.
Please be aware that third-party DLL's are unsupported by ESRI. Continue at your own risk.
Visit the LizardTech website and navigate to the 'Third Party Plug-ins and Updates' section to get more information and download the appropriate file.