Bruce,
LNKxxxx errors are often (though not always) the result of missing libraries or public exports using a correct calling convention. Although you use and call functions defined in Header files possibly requiring an additional Namespace (both used for the compiling), the linker (used to link/merge binary library objects) needs to know what specific libraries are required to fault in required binary code for those functions (Project: Linker > Additional Dependencies).
In both functions used above, both have binary objects located in: library\DgnPlatform.lib; which should be included in most of your "template" Visual Studio projects going forward. The MDL C API help in most cases should provide a "Required Library" remark indicating which library is required to link with. For the MDL C++ API in most cases you can use the method's namespace or direct export type as the hint for which library to link: e.g. DGNPLATFORM_EXPORT ReplaceText implies to link a DgnPlatform lib.
As a side-note, the upcoming MicroStation CONNECT Edition Update 5 SDK will be including a new SDKSearch script that should help make identifying Local and Remote(forum) based resources (references, code, conversations, etc.) for both new development and migrations much a easier task to perform.
HTH,
Bob