When postings answers there is a check box for community Wiki answers. If you are posting a question how can this be made Community Wiki question as such checkbox does not appear?
The current list
Making a question CW is something which moderators (but, now, only moderators) can do (moderators see the CW checkbox available when they edit a topic).
(Until 2010 users could mark their own questions as CW -- see here -- but they changed that, e.g. because it caused arguments, like "your question is slightly off-topic, so, you must mark it as CW").
Examples on other sites
CW is and should be used rarely. It's mostly intended for collaborative topics, where there's not one best answer -- for example,
This is one answer edited 10 times over more than 2 years
This is many, separate answers; with an alphabetically-sorted list of links to each answer edited into the topic's "question".
Or for example there's,
This is so popular that it's CW and locked, edited nearly 100 times, and can now only be changed by their moderators (and, I guess but don't know, might have caused some disagreement if everyone tried to add their favourite book or the latest book to the list).
CW on this site
This site has a few CW topics. They are listed here:
Another example of something like CW is our,
CW is a bit useless if it's not a collaborative effort. If there's a CW topic you'd like to create, perhaps post it here on meta first as a proposal.
Note that this site isn't really meant to be a wiki ... not trying to compete with Wikipedia.
Another time when CW might be useful is when nearly-the-same question is being asked by different people again and again -- because they're nearly the same they ought to be in one place, because they're not quite the same the answer might need to be more detailed/inclusive/edited to account for the various difference.