From where do I Press the PLUS and MINUS Keys?

I would first make sure that your "input method" settings is set to "US" (and definitely not "United States-International"). You can do this through the Control Panel. A restart may be required.

For Windows 7:

1. Open Control Panel
2. Click on Clock, Language, and Region, and then Region and Language.
3. Click the Keyboards and Languages tab, and then click Change keyboards.
4. Under Installed services, click Add.
5. Double-click the language you want to add, double-click Keyboard, select the text services options you want to add, and then click OK.

See also:

• Add or change an input language (archived),
  with video and hidden text (click on “Show all”)
• Language bar missing from taskbar, which appears to contain the same information

For Windows 10:

1. Open Control Panel.
2. Click on Clock, Language, and Region and then click on Language.
3. If you don't see a card reading "English (United States)" in the list, click on "Add a language" on the toolbar to add it.
4. On the same row belonging to the "English (United States)" card, make sure there is a line reading "Keyboard layout: US". If it does not (that's the assumption of this answer), click on the "Options" hyperlink belonging to the same line, to the left.
5. In the "Input method" section, click on the "Add an input method" hyperlink to add a keyboard layout called "US". Delete any other installed input methods pertaining English.

See also:

• Add and switch input and display language preferences in Windows 10

In your case, it likely refers to the + and - keys of the numeric keypad: