The resources folder is located in the module structure at:
In a non-mavenized module, it would be at
A file named taskpane.css in the resources folder for the module formEntry can be accessed via http://../openmrs/moduleResources/formEntry/taskpane.css.
To add it to your jsp page just do the following.
The reason the files cannot be kept in the folder /web/module along with the other jsp files is due to the Spring redirection taking place. The jsp files are accessed either by a redirect from their corresponding .htm file or via setup in the module's application context file.