Sounds more like the Humbrol/Revell type of putty that you get in the UK.
I've just bought some as well, but haven't even opened it yet.
However, if its like the Humbrol/Revell stuff, just queeze some out onto a suitable surface and apply with a small spatula, I use small pieces of plactic card. You can 'thin' the Humbrol stuff with a little plastic solvent, which makes it easier to work with, I'm assuming that you can do the same with the Mr White.
Karl