http://www.microsoft.com/en-us/download ... x?id=40784
Download the ones marked _x86 and _x64, and install them.
This isn't the solution, though - if you didn't have the correct runtime, it'd be much more noticeable (read: it'd cause AviSynth or anything that loads it to crash or otherwise error out, and not with the error message given above). It is, however, a generally good idea to install the stuff anyway. It bewilders me why Microsoft doesn't automatically put the MSVC runtimes into Windows Update as required updates.
Like mirkosp said, it's that the plugin load is somehow not functioning. This could be because the registry got screwed up and autoload isn't working right, or if you're using LoadPlugin to do it, that the location is wrong. Or it could be an issue with mixing up 32/64-bit versions of AviSynth and 32/64-bit versions of plugins and trying to load with a mismatched version. LSMASHSource doesn't have the 2.5 vs. 2.6 plugin issue (to my knowledge), and neither am I aware of 64-bit builds of it floating around, although I could easily be wrong about that (which means, if it is a 32/64 bit issue, you'd be trying to load 32-bit LSMASHSource with 64-bit AviSynth via 64-bit VirtualDub, rather than trying to load it with 32-bit AviSynth via 32-bit VirtualDub). 64-bit VirtualDub cannot load 32-bit AviSynth, just like 64-bit AviSynth cannot load 32-bit plugins.
If it is the registry issue, then reinstalling AviSynth is usually the proscribed course of action. All things considered, that means AviSynth+
(which is the only reason to have a 64-bit version of AviSynth these days, since the 64-bit version of avsplus is the only one that actually has some amount of official development support, and IIRC, the installer automatically provides both 32-bit and 64-bit versions when installing on a 64-bit OS). You'll need to make sure you have the Visual Studio 2012 runtime, so follow the same 'get the _x86 and _x64 packages and install them' procedure mentioned at the top of the post. The 2012 runtime installers are here:http://www.microsoft.com/en-us/download ... x?id=30679