(Disclaimer - I've never installed any of the content, nor do I own either of the games on my client (to my knowledge, unless it was part of some pack I got and never installed it due to me not enjoying that type of game)
Um, ok, installed on your server that way and it installed to the same folders as your other episodic/portal/whatever content (I'm not sure what engine, would guess its orangebox "new" also)
Your server run Windows? Linux? Mac?
Your client runs Mac, right?
Can anyone else running a client the same OS as your server see content?
When you try to spawn any of the items, any errors on client or server?
I remember when client starts up, it tells me what content it's adding of all the 'Source' games I have and what I'm authorized to have. Does your client tell you L4D - added? Does the server have anything like that (been a long time, I forget)?