Please provide additional details.
What extensions are installed, including the version numbers?
You mention that you have eCart 5.0.05 installed, the current version of eCart 5 is 5.0.7.
If you also have Design Extender, Data Bridge, or free Tools, those extensions must be installed after eCart 5.
Are you also installing DEsign Extender, Data Bridge and Free tools? if so, the install procedure would be:
-Install eCart 5.0.7, then open DW and close it.
-Install Design Extender 1.1.8, Data Bridge 1.1.9, Free Tools 1.1.0
-Open DW, if the WebAssist installer does not start, open a php page to force it to start and finish the installation of the extensions. Once the WA INstaller completes, restart DW.