It's actually fairly straightforward; it's just the "nomenclature" in use at Flickr that was my stumbling block, I think -- I don't really think of a sidebar slideshow being an "app" -- and though I'm fully conversant with HTML/CSS, I can't really think of myself as a "developer" -- HTML being primarily a display-formatting language, I guess I'm more of a "formatter" -- a designer, really -- not a programmer. 
Basically, the steps are
1) get a flickr account, if you don't have one already;
2) upload some pics, and put them in a "set";
3) in Mojo, edit a page, and add the feature "Flickr Slide Show";
4) view the page, and you'll see the red text prompting you to enter various details for the feature;
5) click the "monkeywrench" to edit the feature instance;
6) in the blanks provided, enter your Flickr username, and the pixel dimensions you want the slideshow to be;
7) for the API key requested:
a) at the Flickr site, you need to request an API -- to save the time and trouble finding where to do that, go to http://www.flickr.com/services/api/keys/ -- and yes apparently you do want to create an app, if you're requesting an API.
b) choose non-commercial or commercial version -- non-commercial for most stuff unless you're creating it for a site that's expressly dedicated to turning a profit;
c) go ahead and describe in the blank provided what you want to do with the resulting slideshow (or whatever it is you're making -- a slideshow in this case);
d) if you selected non-commercial, your next page will give your API key; in this instance, where you just want to plug it into the Mojoportal feature, you're done -- grab the alphanumeric key and enter it in Mojo in the blank for "flickr API key", beneath the username blank (in the "feature edit" screen for the Flickr slideshow), and once you save, you should then see the slideshow on the page.
e) if you selected "commercial", you're on your own, and probably know so much more than I do about this sort of thing that you really didn't need these instructions in the first place.
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That's the basic process. Not sure how well I described it; I did this late last night when my eyelids were 3/4 lowered, which probably accounts for my initial stupidity in recognizing how simple this really is. If anyone has better terminology for the described actions, please, please feel free to edit -- I can only use the terms I know and recognize, but again -- I'm no developer, so I may not refer to things accurately or appropriately.
Have fun on your mini-vaca, Joe -- the weather here in Atlanta is so totally crap, I could use one myself. 
Andria