First you must be using at least RSLinx Classic Gateway or Professional. Not Lite or Single Client. Lite doesn't support DDE/OPC and Single Client doesn't support FastDDE or 3rd party which you need for Intouch.
Once you confirm you have the right version of linx, you'll need to add/configure the DF1 driver to connect to the SLC serial port. Once you have that configured in Linx and talking, you'll need to create a DDE topic and point it to your SLC in the browse tree. Name the topic something like MySLC or whatever makes sense for your class.
Now, in Intouch, under AccessNames, create a new one, assign the topic name you created above for the name and topic, leave the computer name blank, and in the application enter RSLinx.
Now, when you create your tags, make them I/O, assign the above accessname and give it an itemname that makes sense to your SLC (something like B3:0/0).