You have to use Expressions to achieve this.
Example
Assuming that:
The packages are located in the following Catalog:
MSDB\MyPackages\Import\
The Package Name is stored in the variable @[User::Variable]
Solution
First of all, create an OLEDB Connection manager to connect to the SQL Server instance related to the Integration Services Catalog.
Add the Execute Package Task and configure it To run package from SQL Server , and use the connection you have created.

On the Execute Package Task (just click on the task and press F4 to show properties Tab) go to Expressions.
Set The Package name Expression To:
"\\MyPackages\\Import\\" + @[User::Variable]

Side Note: I assumed that all package are stored in MSDB\MyPackages\Import\ otherwise you have to store the full path inside the variable