Let's see how to add an assembly inside SSIS script task. At first, create a simpe class library without any code and build it. I have copied my dll from the default location to D:\SampleTest\CustomAssembly. Then add a script task to a new ssis package and open the script task's editor and try to add the new assembly from the above path. Following figure shows how to add an assembly in a script task:

After selecting "Add Refernce" option, unfortunately there is no assembly with name  "CusotmAssembly.dll". To make the things more worse, there is no browse button so that we can browse to the path and add the asembly. This is because "Add Reference" option will expose only those dll's which are added to global assembly cache (GAC). An assembly can be added to GAC using gacutil.exe which can be executed from command line as shown:

Above figure suggests that the assembly should be strong named before adding to GAC. Check this link for strong naming an assembly. Once the assembly is given a strong name, try to add it to GAC again and this time it should get added successfully. Now, go and add a reference to the new assembly in script task  and check if the newly added assembly is available/visible or not. If it is not there then copy the assembly to C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727. Now the CustomAssembly.dll should be visible inside script task.

To conclude: Inorder to add a custom assembly to a scrip task or script component in an  SSIS package, we need to

• Strong name the assembly
• Add the assembly to GAC
• Copy the assembly to C:\WINDOWS\Microsoft.NET\Framework\<version>