You need to import the targets file. Took me a while to figure that out as well since it's in the msbuild extensions directory together with a dll. Here's a basic sample of how to use it (note this is for 2012Spring but you get the idea):
<Import Project="$(MSBuildExtensionsPath32)\InstallShield\2012Spring\InstallShield.targets" />
<Target Name="BuildInstaller">
<InstallShield.Tasks.InstallShield
Project="/path/to/my.ism"
ProductConfiguration="Package"
ReleaseConfiguration="MSI" />
</Target>
Btw if this doesn't work out for some reason, you can always invoke ISCmdBld.exe in an Exec task, it will do the job just as fine.