Error: (vcom-11) Could not find work.altera_syn_attributes.

The modelsim_ae normally ships with precompiled versions of the Altera-specific VHDL -- the altera_syn_attributes package was probably just omitted as an oversight. It contains declarations for all the Altera-supported attributes and should be compiled into the altera library. I'd file a support request. In the meantime, you can simply compile the package manually into the altera library, which I'm hoping you can do in modelsim_ae. GuruDog points out the file you need.