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Refer to section 11.2. Obtaining dissection data of the Wireshark Developer's Guide.

Basically, you'll call, where fieldname is the field of interest. In your case, you seem interested in 2 fields, namely frame.number and, so you'd have something like:

local frame_number ="frame.number")
local stream_index ="")

From there, you would obtain all values for this field using (). For the frame number and TCP stream index, there is probably only 1 value for each field per packet [at most], so there's likely no need to worry about multiple values here. After that, you can grab the information you want from it. For example:

function foo.dissector(tvbuf, pinfo, tree)
    local frame_num_ex = frame_number()
    local stream_idx_ex = stream_index()

    # Open a Lua Console to see the value printed.
    print("frame number: " .. frame_num_ex.value)
    if stream_idx_ex then
        print("TCP stream index: " .. stream_idx_ex.value)

Refer to the Wireshark Developer's Guide and to Wireshark Lua and related wiki pages for more information and for plenty of examples.