You can configure an editor by going to PCTeX's Settings menu, selecting Configure Tools, clicking Add Tool, and opening the editor you wish to use.
To tell the editor which file to use, you can probably just select "Selected TeX file" under "Filename Parameter", but you can also use the Advanced Parameters button or just enter the parameters in the "Parameters:" field.
When you're done, click OK to return to PCTeX.
Next, return to the Settings menu, select Default Settings, select the Editor tab, make sure "Automatically check for external modification" is enabled, and click OK.
Now you can open a file in PCTeX, go to the Tools menu, and select the editor. When you return to PCTeX, it should detect the changes and load the new version of the file.