In order to load long texts using transaction LSMW:
Create an LSMW.
In the first step, select “Standard Batch/Direct Input” radio button.
Enter Object 0001, Method 0001. Program name will be filled automatically with the text “/SAPDMC/SAP_LSMW_IMPORT_TEXTS”.
In step 5, assing the long text to both structures.
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This is a nice tutorial about the topic:
http://scn.sap.com/community/erp/logistics-mm/blog/2012/12/28/lsmw-long-text-load-by-direct-input-method