To scroll in a manual iOS Session requires a little more work then scrolling a webpage; There's no scroll bar and you can't press and hold because it selects the text. Instead, you just need to click and hold, but it's a little touchy; Hold too long and you'll select text, instead:
1. Click on the iOS Emulator to ensure it's got focus
2. Press the mouse button, leaving it down and...
3. Drag up or Down
If nothing happens, hold the mouse key slightly longer before dragging. If you get a text selection box instead, click outside the box to dismiss it, then try clicking for a shorter duration before dragging.