The app/design/frontend/[package]/[theme]/ is in fact the same as app/design/frontend/default/default/ if you are using the default template, with /default/ as your “package” and the second /default/ as your “theme”. Since you are here on Hellotheme, I assume you are using an HT template and have followed the instructions on how to install it, so your folder structure should look like app/design/frontend/default/hellococo/ or whatever theme you purchased.
There’s a lot of hype over Facebook like plugins, but I prefer getting my button code directly from Facebook http://developers.facebook.com/docs/reference/plugins/like. One reason is that the like button is for an entire website and usually not for a specific page on your site, so you don’t need it unless you are liking individual products, wherein a plugin might help if it allows this–but do check that it does or you’re not spending time wisely.
Then you can put this code in a CMS static block, and then put that block in your template. Instructions for doing that are basically the same for making static blocks for your template. Magento also has tutorials on that.