There are two kinds of macros, velocity macros and syntax macros, and there are two main document syntaxes available, xwiki/1.0 and xwiki/2.0.

• velocity macros, like #warning, can be used in 1.0 documents directly
• velocity macros must be placed inside a {{velocity}} block in 2.0 documents to work
• syntax macros, like {{warning}}, can only be used in 2.0 documents

Since xwiki/2.0 is the default syntax now, to use a velocity macro you should write something like this:

Note that for this particular case, there's also a syntax macro, so you should write: