Please note that this information is also available in the Quick start Eclipse that you may find in the documentation section of GX DeveloperWeb.
Start Eclipse 
Go to Java /JEE perspective.
Menu-> Window -> Preferences.
Web and XML - XML Catalog
Click on add
Click on the small folder button on the top-right and select FileSystem
Now fill in the following values for example:
Functions tags:
URI: file://<>/webmanager/webapps/backend/WEB-INF/wm/tld/functions.tld
Key Type: URI
Key: http://www.gx.nl/taglib/functions
Wmedit tags:
URI: file://<>/webmanager/webapps/backend/WEB-INF/wm/tld/wmedit.tld
Key Type: URI
Key: http://www.gx.nl/taglib/wmedit
Wm tags:
UR: file://<>/webmanager/webapps/backend/WEB-INF/wm/tld/wm.tld
Key Type: URI
That's it.
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