You'd also need to add a <Directory> entry for /tmp and probably also an alias to the httpd.conf, e.g.:
Alias /tmp/ "/tmp"
Options Indexes FollowSymLinks MultiViews
allow from all
Apache won't serve up a directory unless you allow it too (the user Apache is running as also does need read access to the directory and all the files within it, as worm5252 points out.
edit to add: It's probably a really bad idea to allow users to download stuff from /tmp, because any user (include the one Apache runs as) can write to it. This is a security vulnerability. If you must use /tmp, at least create a subdirectory with controlled access and only allow Apache to serve up the subdirectory.