Windows 8.1 uses KMS protocol v6, which is currently unsupported by Microsoft Toolkit and has yet to be successfully cracked and emulated. Windows 8 uses KMS protocol v5, which is currently supported by Microsoft Toolkit. Microsoft Office 2013 VL uses whichever KMS protocol the operating system is using. Because you are using Windows 8.1, Microsoft Office 2013 is also using KMS v6, which is currently unsupported.
You are going to have to wait for a solution to KMS protocol v6 (could be weeks, months, years, or never), use the solution above, buy Office 2013, or downgrade to Office 2010 (which uses KMS v4 no matter what the OS is).
Edit: With regard to how long it might take to reverse-engineer KMS protocol v6, it took around 6 months to reverse engineer KMS protocol v5. It might be that KMS v6 is so similar to KMS v5 that it will be reverse-engineered much earlier. It also might be that new security measures will make it more difficult to reverse-engineer. Only time will tell.
Edited by Lacius, 07 September 2013 - 12:41 PM.