Isolator++ Professional requires PDB to work correctly (See Download Pdb's) and will first look for the required PDBs in the current folder and in %windir%\Symbols\dll.
If it does not find them, you can turn on the option to download them from Microsoft Symbol Servers.
This is required if you fake microsoft classes or if your methods return microsoft types.
To do that open the Isolator Configutator in the install directory
and Check "Use Microsoft Symbol Servers"
You can now set a path for cached PDB files via IsolatorConfigurator:
Edit box "Cache symbols in this directory" and set a path for local cache debug symbols.
Click Apply to save configurations.
Please note that it may take a few minutes to download the required PDBs into the selected folder.