What is likely happening is that you did not install the SSIS components -- they are an installer option.
OR, you installed the SSIS components, but only the 64-bit version. Since SSDT is 32-bit only, the SSIS components will not load in the designer if the 32-bit version is not also installed.
SSIS components are not installed by default, because you don't strictly need them to model metadata, build, etc. Those components are only necessary if you want to run or debug the generated packages, so most customers only install them on a subset of developer workstations and on the SSIS server(s).
Thank you.
Varigence Support