You'd be well-advised to download the VPN Configuration guide before attempting to set up a PPTP tunnel, because some of the settings aren't obvious.
Figure 4 shows the PPTP setup screen you reach after selecting from a list of 20 possible PPTP "accounts" on a previous screen. Folks who've set up a PPTP tunnel will know what to do with most of the entries, but the "host" checkbox isn't what it seems (it's checked only if you're connecting to a Windows PPTP server), and setting up a PPTP server requires you to set the Gateway IP to 0.0.0.0. (all this is explained in the VPN Configuration guide). You'll also have to scroll down and set the IP Address Pool on the page that lists the PPTP "accounts" if you're setting up a PPTP server, something that I think should be done automatically based on the Plus' DHCP settings.
Note that the 20 accounts doesn't mean the Plus can handle 20 PPTP tunnels. Although I couldn't find a spec for the number of PPTP tunnels in any of SMC's documentation, the Status screen indicates that a Plus configured as a PPTP server can handle 2 client connections, and can handle one connection to a remote server when configured as a PPTP client.
Debugging and managing PPTP connections is also done through the Status screen as you can see in Figure 5.
But this time, it looks like there was more attention to design details because Connect and Disconnect buttons automatically appear for each client connection, along with a Disconnect button for each server. You also get PPTP connect and disconnect logging in the Security Log window which, although terse (and a pain to read due to the small, unsizable window), at least gives you some insight into what is happening during PPTP tunnel setup!
Tip: The PPTP VPN capability uses a Client / Server model, which automatically sets up a PPTP tunnel from client to server only. This probably isn't a problem in real life, since the Plus seems to be pretty aggressive in setting up and keeping a tunnel running. So it's likely that the tunnel will be up if a LAN client on the server side of the tunnel wanted to talk to a machine on the client side.
Tip: The Plus doesn't automatically disable one flavor of VPN tunnel when another is disabled, or give you a Connect / Disconnect button for IPsec tunnels. You must disable the IPsec tunnel (or entire IPsec feature) in order to kill an IPsec tunnel.