If you pull down the notification bar at the top of the screen, there should be an option to mount usb storage, that will let you view the phone as an external drive on the PC.
To install apps, there should be an app called Market (there might be 2, an orange branded version and an official android one, it's the official one you want). Search for K9 or Maildroid on there, hit install and it will do all the work for you.
Alternatively, there's a web based version
here which is much easier to search with than the version on the phone. Sign in with your gmail account and you should be able to install apps via the web on your handset (providing the handset has a network connection).
The link for K9 is
here and the link for Maildroid is
here.
As an aside, the handset is much better once you install an alternative version of Android, it's not bogged down by all the Orange junk that comes pre installed so once you're a bit more comfortable using the phone, take a look at the stickies
here if you want more info.