It's not placeable within the editor. One of the Extended Editor add-ons might have it though.
As far as searching is concerned, try more generic searches. Something like "streetlight" instead of "placing streetlights in editor" would be better. Even just "streetlight editor" would be an improvement.
A search on the BIS forums for "streetlight editor" results in this thread:
http://forums.bistudio.com/showthread.php?t=35694&highlight=streetlight+editorWhich indicates that at least for ArmA, you needed an editor update to access this object and even what command to use to turn it on or off. Here's a link to one of the ArmA2 editor updates:
http://www.armaholic.com/page.php?id=5932 I tend to avoid them since it's likely that to add this custom streetlight where you want it, you're going to require all your players and any server this is run on to also be using the editor update as an AddOn. You can fake streetlights better using light objects.
A search on the BIS forums for "placing streetlights in editor" results in 2 results, one of which is a 5 page thread where you'd have to dig to find the answer.
Searching for keywords not phrases and it'll do much better for you.