First update the catalogs using:
apt-get update

Then use apt-cache to view the available packages. Note that you will see alot more packages that the ones being shown in the app manager. To ease the search of a certain package you can use:

apt-cache search <keyword> |less
this command will use the pager less to show the output of apt-cache search i.e. you will be allowed to browse it and search.

You can try the faster app manager it has nice features such as selecting multiple applications and installing them.