When in-game, press escape.
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Options > Open to LAN
It is possible to play Minecraft without accelerated opengl mode without any mods. Simply open the options menu and choose video settings. Within there is an option to enable or disable advanced opengl.
If you open the menu, then click on GUI scale, it will adjust the size of you inventory
What I think you're referring to is the mod for Minecraft "Too Many Items" if you download and enable the mod it should be very easy to do. To simply open up the "cheat box" or menu press the I or E key to open your inventory.
Go onto Google, type toomanyitems 1.7.3, click the link for the Minecraft Forums. Download it and open it. Next, you need to open the start menu, then search %appdata%. Open up you're .minecraft folder, and then open the bin folder. Then right click on the minecraft.jar file, and then open with -> Winrar archiver. Then just drag the files from the folder you downloaded into the folder you just opened. Then close both, then open minecraft.