Just Click on "Cancel" on the Pop-up window and then choose SSV2 method. Also if the original Unmodified BIOS has a Bad checksum so will the Modded Bios. Many Original Bios Files with Multiple Languages have a Bad Checksum for whatever reason. But they still mod and work to activate Win 7 in most cases.
Actually there's a little way around the bad checksum too. Essentially you fill all the fields, slic slp blah blah, but instead of Go, hit verify. It will correct the bad checksum.
Does it still Flash and properly add Slic 2.1 that way. I have done plenty with a Bad Original Checksum and just ignore that and they have all worked so far. There is also an Advanced Option to correct the checksum but since the Original is Bad, I don't use it. PS The SSV2 Mod on the Above BIOS requires the /p /n /c switches to properly flash the NVRAM region which now contains the SLIC Table.