In the Preferences, click the "Launching" tab, then choose the "Default Application to Edit Photos" and choose "Assign Application..." I assigned Adobe Photoshop CS3, but you can choose Lightroom if you want.
Then, when viewing a contact sheet, select the photos you want to edit in Lightroom (Or ACR), and press Command-E on a Mac, and they will open in Lightroom (or ACR). If you are viewing a large proof, just hit the "e" button (no Command-) and it will open that individual image.
Photo Mechanic is more than just a quick downloader and viewer. It's also (in my opinion and experience) the best and quickest metadata editor. All the crucial metadata edits -- changing the filename, adding complete caption information, and complete keywords -- are quickly and easily accomplished, once you have set up your templates for doing so.
I will happily concede that the PM manual is something of a joke. But if you can work your way through it, it can pay off. One nice feature is that you can save snapshots of frequently used dialog boxes, so you can instantly re-use them. This works for Ingest, IPTC, Rename, Save As (which has a ton of features), and pretty much any other dialog. Note that you can use variables in almost every dialog, too, which is also quite handy.
I suppose I could go on and on (maybe I should be writing the guidebook...) Just play with it for a while and see what works. If you have any specific questions, just ask.