set global variable parameter?
You can set any global variables to any value on the fly using below statement in MySQL prompt:

set global &lt;variable&gt; = &lt;value&gt;;


In your case use the following to turn on slow query log:

 set global slow_query_log=1;


By default the "slow wait time" is set to 10 sec, but you can change it as well.

Hope it helps.