In-place completion is enabled by default in the IDE. If you prefer the modal dialog style of completion familiar to users of LispWorks 5.0 and previous versions, deselect the Use in-place completion option.
When using in-place completion to complete a filename, by default you must always select an item from the in-place completion window. You can accelerate this interaction by checking the option Auto-insert on single file completion . Then, if there is just one possible completion, it is automatically selected and appended to your input.
LispWorks IDE User Guide (Macintosh version) - 25 Nov 2011