There is no need to bother with telling SlickEdit about an executable or a makefile for code browsing.
Create the workspace/project by following these steps:
Project->New
Under C++ choose "Other C++" (even if your project uses gcc or clang, choose "Other C++" anyway, this is important to avoid requiring executable or makefile)
In "Location:" choose the root directory of your source tree
In "Project Name:" choose a name like "asterisk"
Make sure "Create new workspace" is selected.
Click "OK"
Now you get a new dialog. Click on: "Add Tree"
In next dialog, navigate to the root of your directory tree
In "Include filespecs" choose a set of filespecs that will capture all of your C++ source files, such as one with common *.c,*.cpp,*.h, etc. extensions
Make sure "recursive" is checked
Now click OK to scan the tree, you should see all your source files populated in the "Files" tab.
Then in Project Properties dialog, click OK again to exit.
This will create your project containing all your source files.
Now you will be able to code browse.
Go to a symbol you want to see the definition of and use pulldown menu (or keyboard shortcut):
Seach->Go to definition (or Ctrl+.) to find the definition only, but not references
Search->Go to reference (or Ctrl+/) to find all references (see the "References" tool window at bottom with double-clickable results)
After you navigate to definition or reference, you can press Ctrl+, (Ctrl+comma) to go "back" to your original location.
You can also choose the "Preview" tab at the bottom, then put the mouse on any symbol in a regular editor window with your source file and you will see in the Preview window where that symbol is declared/defined.
Good luck!