Definitely check out the documentation in the Numisdoc section.
Here's a quick primer, though, as the available documentation might not cover the new format.
Step 1: Find the coin you want to add to your collection. I usually just use the basic search function on the Catalogue page.
Type a few descriptive words and the coin's date and hit return. If that doesn't turn up what you're looking for (or turns up too much), you might need to use the advanced search function, which allows for finer-grained searching.
Step 2. Select your coin from the search results. This will take you to the page for that coin. Click the "enter" icon for the particular year and mint for your coin (in this case, I want to record that I have the 1822A 1 centesimo; the red arrow points to the "enter" icon for that year/mint).
Clicking the "enter" icon opens a pop-up window. That's where you enter (1) the coin's grade (just click the appropriate button), (2) how many you have (default is 1), (3) how much you bought it for, (4) any notes you don't want shared with the rest of the Numista community, (5) whether you wish to exchange the coin (default is no), and (6) any notes you do want shared with the community (I typically reserve that field for notes about coins I'm swapping). None of the fields is required.
Click "Save" if that's the only example you have of this coin. Click "Save and add again" if you want to add another (e.g., if you have other years/mintages, have the same coin in a different grade, or have one that you'd like to swap).
That's it. You've added a coin to your collection in Numista. Once you click the "Save" button, the underlying page will update reflecting the info you've added: