Riolist2 is exactly right. You can also add everything on the Slot 2 card by selecting Files and then long-pressing Play while <microSD1> is highlighted. Then choose Curren Playlist - Insert Shuffled.
You can also tell Rockbox where to start the File browser. Riolist, it appears yours starts within the Music folder; you can clear this setting by long-pressing Play when Files is highlighted in the main menu and scrolling to the bottom of the options to "Start File Browser at /". To set a specific directory, highlight the directory, long-press Play, and choose "Start File Browser Here". You can also tell Rockbox which folder contains you Playlists in the same way, otherwise it defaults to a folder named "Playlists" in your root directory.
Information dump follows:
About how Rockbox treats files: everything is Playlist based. When you select a single track in a folder, Rockbox will automatically create a playlist that includes every music file within that folder and play through them according to your settings (shuffle, repeat, etc.). You can add tracks, remove tracks, rearrange them, etc. and even save the current playlist if you like it.
So if you want to Shuffle all tracks on your player, you have to add everything to the current playlist. Riolist's solution is the simplest: store all your music folders within one parent folder, and when you want to play everything, select the parent folder, long-press Play, select Current Playlist - Insert Shuffled. If you don't add new music often, save this new shuffled Playlist as "Everything" so you can use it later.
If the player is currently playing something, your music will be shuffled into the current playlist. If the player is stopped (not paused, but stopped; you can stop playback by pressing the power button), Rockbox will start fresh and only include the music in the selected folder.
When you select "Insert Shuffled", Rockbox creates a randomised Playlist, and then plays straight through it. If you select "Repeat All", when it reaches the end of the randomised Playlist, it will start over and play your randomised tracks in the same order as before. This is handy, because there are never any repeated tracks when Shuffled (unless you have several instances of the same song in your collection). If you wish to listen to the same Playlist but Shuffled into a different order, then on the While Playing Screen, long-press Play to call up the Playlist menu, and select "Reshuffle" at the bottom of the menu.
