There is a BUG for the macOS version of Scilab 6.1; After installation and restart you get
the error:

*** CelestLab is not available for this version of Scilab (Scilab 6.1.0 or less required)

This because Scilab 6.1 for macOS returns 611 (6.1.1) of CL__scilabVersion() but
celestlab.start doesn't except numbers greater than 610.

--------

WORKAROUND FOR USERS

1) Install celestlab via Atoms
2) Go to Mac's Finder and open the Application folder
3) Search for the Scilab-branch-6.1 app bundle
4) Right click (control-click) on it and click "Show Package Contents"
5) Go to folder "Contents/MacOS/share/scilab/contrib/celestlab/3.4.0/etc"
6) Open file celestlab.start in a texteditor (e.g. TextEdit) and go to line 27
7) Change 
      if (vnum > 610)
   to
      if (vnum >= 620)
8) Save celestlab.start
9) Restart Scilab
10) Done