I don't remember the installation screen, but one of the following options should work:
- Press 'F3' to enable input on the installation menu. Now type 'vga=normal' and start the installation.
- Press another F-button for installation options (like screen settings, installation source, etc) and change to textmode.
- Run 'Manual Installation'. It's a bit more tricky than the others, but should be completely in text-mode.