The question is a little confusing but then, so is saving Blender files on a Mac - at first
Saving a Blender file:
Just go to the File menu and choose Save or Save as. The File Browser seems rather different to the typical Mac windows but it’s not that different once you understand it. To navigate the file system, you can click on the “…” near the top of the file/folder list and this will take you through parent folders. To navigate forwards, just LMB on a folder name.
Enter a filename in the lower text panel (the one with the cancel button at the end). Hit Enter or click “Save as”.
When you restart Blender it will start with the default screen (it sounds like yours uses the monkey). To continue with your old file, go to File>Open Recent and your file should be listed. If not, choose file open and navigate to where you saved it. To open a file, click on it (LMB or RMB) and hit Enter or just MMB on it (Option-LMB if you don’t have a 3-button mouse).
*Note: You may find the file listings make more sense if you navigate backwards as far as you can then go forawrd from here. Your Blender folder will/should be in the Applications folder. You might want to save files in here but it’s not the best place (I made a Blender’s File Folder called “BlenderStuff” in my user folder and I save everything within folders in there).
It really does take some getting used to for Mac users not familiar with DOS/Unix type filing systems.
Render an image with F12 then once it’s rendered, save with F3. Use the file browser to navigate to your preferred folder. You’ll eventually find the file browser is probably already where you want it to be anyway. Enter a filename in the lower text panel (the one with the cancel button at the end). Give the file an extension (.jpg or whatever filetype you selected in the output format F10 panel). Hit Enter or click “Save as”.
If you don’t specify a filename and path on Mac OS X, the animation file is saved as a file in a hidden tmp folder and called something like “0001_0250.mov”. To find it you’ll need to do a system search for the hidden folder as follows:
Search for: NAME + IS + “tmp”
Hit the PLUS sign for a second criteria selection
Choose: VISIBILITY + INVISIBLE ITEMS
You should see one or more folders. Open them and one should (hopefully) contain your missing file.
For future reference, since filepaths aren’t exactly second nature to Mac users (C:\ means nothing to us) you should choose somewhere to save your files. You can do this on a file by file basis by using the Render panel in the F10 window (left side). LMB on the small folder icon to bring up the file browser and navigate as described above.
You can set a default Render location (recommended) by saving it as part of your user preference. To do this, start with a fresh Blender Scene (File>New>Erase All). Go to user prefs (drag down top of 3D window), go to “File Paths” and use the folder icon to navigate to the desired folder. Put the window back then CTRL-U to save the setting.
Hope this clears it up a little. If not, please explain the exact problem in some detail. In the meantime, take a look at the PDF in my sig below. It explains it with pictures and hopefully that, together with my info, should get you going.