When importing elements with ModelBuilder the ModelBuilder Summary dialog shows the following warning.
The ModelBuilder Summary warning "direction is ambiguous" appears when the model contains valves or pumps that don't have anything specified in the "Downstream Pipe" field.
This warning will appear for nodes missing that field even if the nodes are not being attached to the pipes imported with ModelBuilder.
If you receive this warning while in the process of building a model, then you should just note that the valve and pump elements will need to have this field specified at some point.
Once all of the model's elements are imported, and these nodes have their downstream pipes attached, then you can use ModelBuilder to update the "Downstream Pipe" field from the Shapefile (if this field is included). Or, you can manually select the correct downstream pipe. Use the Validate tool to identify all cases. All offending cases can be highlighted by right clicking on the related user notification and choosing "select all elements with Message ID ..."
Importing external data using ModelBuilder - Building and updating