There's a minor incompatibility between 1.2 library projects and 1.5 libraries. This is easily fixed (all you need to do is set two project properties) ...
1) open the Project properties page and select the Application tab.
2) Set the project type to Library (it will be Flex for a 1.2 library). The project will reload.
3) Then save the project and close it.
4) Re-open the project and select the Classes tab (this should now be visible).
5) Check 'Select classes'. Save. Then check 'Include all classes' and save again. You should now have a 1.5 library project.
If you've still got trouble, unload the project file and make sure that the top part looks something like this:
<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="3.5">
<Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
<Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
The key bits are the AmethystProjectType and the IncludeAllClasses properties. Obviously, you can select individual classes to be included by using the tree in the Classes tab, but this procedure should get you going with Amethyst 1.5.