rtv carries out batch-mode simulations in a multi-step process. To do this, it first compiles the Maia source with mtv, and it then uses your simulator to compile any HDL sources required, and to run the simulation. rtv is driven from a configuration file (simulators.conf), which you'll find in the top level of the Maia distribution.
The configuration file is essentially a list of the steps required to run a batch-mode simulation on any given simulator and operating system. To use rtv, you should first check the configuration file to find out if it supports your simulator, and you should make a note of the name used for your simulator (the name is the first word in each section; search for 'my-simulator' for more details). You should also check Supported Simulators to confirm that your selected simulator is usable. If your simulator isn't covered in simulators.conf, then you can simply add the required steps in a new section in that file.
There are two ways to tell rtv which simulator to use: you can either set the RTV_SIMULATOR environment variable, or you can use the --sim command-line switch. If you're planning on using Icarus for Verilog simulation, for example, then you can set the required simulator as follows:
$ export RTV_SIMULATOR=icarus
On the other hand, if you're planning on using ModelSim for VHDL (in other words, mixed-language) simulations, then you can set the required simulator as follows:
$ export RTV_SIMULATOR=modelsim_mixed
rtv can't set up the required simulator for you; you must confirm yourself that your simulator executables are in the search path.
If you're always going to be using the same simulator, then you should instead modify the supplied settings.sh or settings.csh file, at the top level of the distribution, to change the default export RTV_SIMULATOR command.