DVT VHDL IDE User Guide
Rev. 24.1.7, 9 April 2024

35.41 I know that file.foo is present in the project location, but I can't see it in the Navigator View

Check if there is a Navigator filter enabled preventing the file from being displayed. Bring down the Navigator View Menu and select 'Filters':

Make sure that no pattern in the list matches the name of the file (like for example *.foo).