If you installed a hacked client or mod and it will not show up in the Minecraft launcher, hopefully the steps below will solve your problem. Let me know in the comments if it still won’t work for you so I can (hopefully) resolve the problem.

1. Your clients are stored in the %appdata%\.minecraft\versions folder.
2. To get to this folder on Windows XP/Vista/7 open the start menu, select "Run" and enter %appdata%. Navigate to .minecraft > versions to get to your clients.
3. Alternatively, to get to this folder on Windows 7, 8, or 10, open the search menu (which is part of the start menu in Windows 7) and enter %appdata% in the search bar. Click on Roaming and navigate to .minecraft > versions.
4. Make sure your client folder has a unique name (don't name it 1.7.10 or the name of another client).
5. Your client folder will contain two files, a .jar and a .json. They all need to have the same name. For example, if you are installing WeepCraft, your folder would be named WeepCraft, and it would contain WeepCraft.jar and WeepCraft.json.
6. Once you've made sure of these settings, close and re-open your Minecraft Launcher.
7. Search through your versions list in the launcher profile; it could be further down in the list than you might expect.
8. If it's still not there, go back to your versions folder. Open up the folder that contains the .jar and the .json of the client.
9. Open the .json (for example, WeepCraft.json) file with a plain-text editor. This would mean Notepad, Notepad++, Text Editor, etc. depending on your Operating System.
10. Look for a line starting with "type".
11. If it isn't "type": "release" (it could be "type": "snapshot", for example), change it to "type": "release". Make sure the quotation marks are straight; curly ones (for example, “type”: “release”) won't work.
12. Look for a line starting with "id".
13. Make sure to set it so "id": "name" where name is the same as the name of the folder and its files (for example, "id": "WeepCraft"). Check carefully for capitalization and spelling mistakes. Again, make sure the quotation marks are straight and not curly, because “id”: “WeepCraft” won't work.
14. Once you've made sure of these settings, close and re-open your Minecraft Launcher.
15. Search through your versions list in the launcher profile again, the release (for example, release WeepCraft) could be further down in the list than you might expect.
16. If these settings still don't work, note that some cracked launchers don't support modded clients. If you're not cracked, I can still help you - just leave a comment below.
17. Or, try a different hacked client (I recommend Impact) and see if it works.

Pro Tip: Use the Older Launcher

I’ve been notified recently about all of the drama surrounding the 1.12 update. It seems that in a massive effort to reduce mods and clients for Minecraft, Microsoft (who owns Mojang) has been issuing DMCA notices for custom clients as well as pushing updates to their launcher to prevent launching non-official clients. This whole tutorial and all of the clients we offer rely on the older, but still official launcher, which you can download here (requires Java).