I think you are looking for something like this:
(setq yas-snippet-dirs
'("~/.emacs.d/snippets/"
(file-expand-wildcards "~/.emacs.d/elpa/yasnippet-*/snippets"))
But I think you don't need it...
For load-path
variable... package-initialize
should update it automatically, so this would not be needed.
For yas-snippet-dirs
installed with elpa... just doing M-x yas-global-mode sets the value of that variable just to what you want.