OK, intermixed questions. I'll try to separate them out.
1) problems with download P2 from plogue.com during spec installation.
Lately there have been problems with accessing plogue.com from certain ISPs. For instance, I can't get there at all from work, but can from a T1 circuit at work or from home. I'll alert Seb @ plogue.com, and you can contact your ISP but that's about all we can do.
As you mentioned, you can manually try to download P2 from this link:
http://www.plogue.com/bidule/latest/PlogueBiduleP2.zipunzip it and copy it to your \Plogue\Bidule folder (under program files or program files x86, depending on your OS) and use that file to launch plogue.
2) libcurl missing.
Plogue itself shouldn't need libcurl but spec does, so you would get that error when loading spec, not when starting plogue?
This has nothing to do with visual basic. Neither Spec nor Plogue use visual basic.
The Spec installer should have copied the libcurl dll file from the install bin folder to \Plogue\Bidule
A screen shot of the installer window would be helpful to debug what went wrong. Did you run the installer with Administrator privileges?