Hotmail addresses are not POP based. You can use Hotmail Popper to check Hotmail as if it were a POP account, though.
Download 
Hotmail Popper.
- Open the zip file and extract the one file that's in it.
 - Open the file you just extracted, installation will begin.
 - Click Next
 - Choose program folder, click Next
 - Choose when to start Hotmail Popper (both enabled recommended)
 - Click Next
 - A flame icon will be added to the taskbar (next to the clock)
 - Right click that icon
 - Choose Help
 - Scroll to How do I use it, and follow the instructions
 
- Now, to set up the checking in Messenger Plus!:
 - Go to the Preferences, POP mail section
 - Enter your email address
 - Enter your username (part before @hotmail.com) and password
 - Enter 127.0.0.1 as POP3 server
 
Basically, that's how you can set it up.
The Free Yahoo! E-mail service does not provide POP access.
You could try to use YahooPops, which you can download from 
http://yahoopops.sourceforge.net. That site also contains detailed instructions on how to set it up (not with Plus! specifically, but the details are pretty much the same).
I don't use Yahoo! myself, so I can't tell you more.