You can find all currently installed collar apps in this menu.
A collar that just came out of the box usually features at least the Titler app. If your collar supports a functional Bell or a Label, their corresponding Apps can be found in this menu as well.
Apps can be acquired at The Temple of the Collar™. Some are for free and others are available for a small fee of L$100 to support the coders who write and maintain the particular app.
To install your App simply rez it on the ground or wear it. When prompted with the Installation request choose "Yes" and wait until receiving an "Installation complete" notify in the Nearby Chat.
If nothing ever happened. Make sure that your collar is modifiable*, that the land you are on allows active scripts and touch the App Installer box once again to re-initiate the installation.
NOTE: Some Apps, such as Spy for example, will require full perm or "no copy" collars to be rezzed on the ground first to be able to perform a successful installation.