Yes, you have to watch for that installation directory, if you have a drive D, for whatever reason, and the special install options dialogue is small and difficult to see, but yes, if you click it you get the choice to change the D to a C. Did you change anything else other than just the D to a C?
Also, are you struggling with internet access when trying to get the 7 beta download, in the multi instance manager, or somewhere else, later or sooner?
As a matter of interest, as pointed out to me by someone else in here, there is a version 5 in the list, at least now, which is not Beta, so that may be a better choice for, if and when, you solve the internet issue. There are a couple of small issues that are nothing more than irritations in the 7 Beta, which are not evident in 5.
Anyway, I did not suffer from a lack of internet connectivity in the multi instance manager, having changed the installation directory to C drive.