What OPC server are you using?
From the RSLinx help files:
[quote]ControlLogix 5550: addressing syntax
The format for solicited DDE/OPC items for the Logix 5550 processor is as follows:
For globally-scoped tags, use your user-defined tag names as the item names.
For program-scoped tags, use the format of Program:ProgramName.TagName for the item names. For example, if the program name is Power and tag is Output, the format for the item is Program:Power.Output.
The addressing syntax for a ControlLogix String UDT is STRING.DATA,SC# (where # is the number of characters). For example, use STRING.DATA,SC10 to read the first ten characters of STRING.DATA.
The addressing syntax for ControlLogix I/O bits is [Local|Remote]::[I|O].Data/bit. For example: Local:2:O.Data/1.