D3D9 is 18 years old at this point, so it's going to be difficult to find resources and knowledgeable people to help you out here. My memory is pretty fuzzy myself so I can't really give you specifics either, but I will try to point you in the right direction.

• There's not just going to be a 1-line change to your C++ code to provide your bone matrices as a texture, that's unfortunately not how it works, You're going to have to learn how to create a dynamic texture ("dynamic" here means the CPU can update its contents), lock it, copy in the bone matrices, unlock it, and then bind it to a texture slot for your vertex shader.
• If you're using old-school shader model 3.0 HLSL then you can't use the shader model 4.0+ syntax for sampling and loading textures, which is what you seem to be doing. Just like any other SM 3.0 shader code you'll want to declare a sampler1, and pass it to tex1Dlod to sample from it (you need to use the variant that takes an explicit mip level since vertex shaders can't automatically compute a mip level for you. But it doesn't really matter in this case since your texture won't have mips, so you would pass a hard-coded mip level of 0).

You can also consider solving this problem by splitting your mesh into multiple parts, with each part using less than your maximum supported bone count.

Why do you even need DirectX 9? Now any video card already supports 11. And it is much easier to learn thanks to more complete documentation and has more features.