Problem: Custom tools do not appear in the tree view of ArcGIS 8.1
Custom tools built for ArcInfo 8.0.1 and 8.0.2 do not appear in the ArcToolbox tree at 8.1.
Custom tools built before 8.1 will not be displayed in the ArcToolbox tree because the mechanism for creating custom tools has changed.
Solution or Workaround
Update the custom tools created in ArcToolbox 8.0.1 and 8.0.2 to support the IArcToolboxTool interface. Refer to ArcGIS Desktop Help > Contents tab > ArcToolbox > "Adding a Custom Tool" for a complete discussion of how to implement a custom tool.